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Thesis for the degree of Doctor of Philosophy, Sundsvall 2010
Soil and stream water chemistry in a boreal catchment –
interactions, influences of dissolved organic matter and
effects of wood ash application
Sara H. Norström
Supervisors:
Professor Ulla Lundström
Assoc. Prof. Dan Bylund
Department of Natural Sciences, Engineering and Mathematics
Mid Sweden University, SE-851 70 Sundsvall, Sweden
ISSN 1652-893X,
Mid Sweden University Doctoral Thesis 99
ISBN 978-91-86694-04-3
Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall
framläggs till offentlig granskning för avläggande av filosofie doktorsexamen
fredag den 26:e november 2010, klockan 13.00 i sal O111, Mittuniversitetet
Sundsvall. Disputationen kommer att hållas på engelska.
Soil and stream water chemistry in a boreal catchment –
interactions, influences of dissolved organic matter and
effects of wood ash application
Sara H. Norström
© Sara H. Norström, 2010
Department of Natural Sciences, Engineering and Mathematics
Mid Sweden University, SE-851 70 Sundsvall
Sweden
Telephone:
+46 (0)771-975 000
Printed by Kopieringen Mid Sweden University, Sundsvall, Sweden, 2010
ii
Soil and stream water chemistry in a boreal catchment –
interactions, influences of dissolved organic matter and
effects of wood ash application
Sara H. Norström
Department of Natural Sciences, Engineering and Mathematics, Mid Sweden University, SE851 70 Sundsvall, Sweden
ISSN 1652-893X, Mid Sweden University Doctoral Thesis 99; ISBN 978-91-86694-04-3
ABSTRACT
Two small bordering catchments in Bispgården, Central Sweden, were
investigated in regard to soil solution and stream water chemistry during the frost
free seasons of 2003-2007. Both catchments were drained by first order streams,
Fanbergsbäcken and Gråbergsbäcken, and in Fanbergsbäckens catchment an
extensive investigation of the soil and soil solution chemistry was conducted by
lysimeter and centrifugation sampling. The area of intensive soil solution
investigation was situated in a slope towards a stream incorporating a recharge
area, with podzolic soil, and a discharge area close to the stream with an arenosol
soil. Samples were continuously taken in both the recharge- and the discharge area
of the slope, and stream water was sampled in the streams of both catchments. The
main variables of interest of the study were the interactions, the influence of
dissolved organic carbon and the effects of wood ash application to soil solution
and stream water.
The natural variations and the interactions between soil solution and stream
water were monitored during 2003-2004. In soil solution, most of the investigated
substances tended to increase during the growing season, due to weathering and
microbial degradation of biota. Ca, Mg, Al and Fe were highly associated to
dissolved organic carbon (DOC) throughout the catchment. The low molecular
fraction of DOC seemed to have a higher impact on the soil processes in the
recharge area, while high molecular DOC was more important for transport of
cations in the discharge area and the stream water.
The concentration of different substances in the two streams differed
significantly, even though the catchments were similar in size, shape and
forestation. The seasonal patterns of most of the substances measured were
iii
significantly correlated between the streams, however. Cations and pH correlated
well with DOC and flow. The flow pattern driven by precipitation seems to be the
driver of the stream water chemistry.
Wood ash was applied at a dosage of 3 ton/ha to one of the catchments in the
autumn of 2004, to investigate the initial effects on the soil solution- and stream
water chemistry. WAA is recommended by the Swedish Forest Agency to
counteract acidification in soil and runoff that may be caused by an intensive
biomass harvesting. The impact of the WAA was studied during 2005-2006.
Compared to the control temporarily higher concentrations of K, Ca and SO4 were
observed in the soil solution of the ashed area. In the stream water the effects of the
WAA were easier to distinguish due to higher sampling frequency. The strongest
effect was seen for K, but increases in the stream water were also noted for DOC,
Ca, Mg, Si, Cl and malonate. No increase in pH could be statistically verified
however, and overall the initial effects of the WAA seem mild.
Keywords: Boreal forest, catchment studies, dissolved organic carbon, soil solution,
stream water, wood ash application
iv
SAMMANFATTNING
Två angränsande avrinningsområden i Bispgården i centrala Sverige
undersöktes under den snöfria säsongen 2003-2006. Båda avrinningsområdena
dränerades av första ordningens bäckar och i det ena, Fanbergsbäckens
avrinningsområde, gjordes en omfattande undersökning av mark och markvattenkemin. Markvatten provtogs genom centrifugering och med lysimetrar.
Provtagningen gjordes i inströmningsområdet där jordmånen var en typisk
podzol samt närmare bäcken, i utströmningsområdet, där jordmånen var en
arenosol. Bäckvatten provtogs i båda avrinningsområdena. Interaktioner mellan
mark, markvatten och bäckvatten undersöktes med särskilt fokus på inverkan av
organiskt kol samt påverkan av askåterföring.
De naturliga variationerna i markkemin samt interaktionerna mellan mark- och
bäck vatten undersöktes 2003-2004. I markvatten ökade merparten av de studerade
ämnena under provtagningssäsongen beroende av ökad vittring och recirkulation
av biota och fallförna. Ca, Al och Fe var i stor utsträckning associerade till löst
organiskt kol (DOC). Den lågmolekylära fraktionen av DOC hade en större
inverkan på markprocesserna i inströmningsområdet, medan den högmolekylära
delen var viktigare för transport av katjoner i utströmningsområdet samt i
bäckvattnet.
Koncentrationsnivåerna av olika substanser i bäckarna uppvisade skillnader
trots avrinningsområdenas yttre likheter med avseende på storlek, form och
beskogning. Trots de kvantitativa skillnaderna erhölls emellertid liknande
säsongsvariationer i koncentrationerna, vilket indikerade att kemin i huvudsak
styrdes av mängden DOC som i sin tur berodde av avrinningen som drevs av
nederbörden. Höga halter av Ca, Mg, Al och Fe återfanns associerade till
högmolekylärt DOC i bäckvattnet i en utsträckning som inte rapporterats tidigare.
På hösten 2004 spreds 3 ton aska/ha till Fanbergsbäckens avrinningsområde för
att undersöka de initiala effekterna på mark- och bäckvatten kemin. Askåterföring
bör ske minst en gång per omloppstid vid helträdsavverkning, i enlighet med
Skogsstyrelsens rekommendationer, främst för att motverka försurning i mark och
avrinnande vatten som antas uppstå vid intensivt uttag av biomassa. De initiala
effekterna av askåterföringen på mark- och bäckvattnets kemi studerades under
2005-2006 och Gråbergsbäcken kunde användas som obehandlad kontroll vid
undersökning av vattenkemin i Fanbergsbäcken. I undersökningen av markvatten
v
återfanns stora säsongsvariationer, vilket gjorde det svårt att urskilja eventuella
effekter av askåterföringen. Tillfälligt högre värden av K, Ca och SO4 återfanns
dock i det askade området i jämförelse med kontrollområdet. På grund av högre
provtagningsfrekvens i bäckvattnet var det lättare att påvisa förändringar i
bäckvatten kemin. Framförallt märktes en signifikant ökning av K jämfört med
kontrollbäcken. Sådana ökningar, om än inte lika markanta återfanns också för
DOC, Ca, Mg, Si, Cl och malonat. En tendens till ökat pH kunde observeras, men
denna kunde inte verifieras statistiskt. De initiala effekterna av askåterföringen var
således till synes milda, och den eftertraktade pH-effekten erhölls ej i denna
undersökning.
vi
TABLE OF CONTENTS
ABSTRACT ...................................................................................................................... III
SAMMANFATTNING.................................................................................................... V
LIST OF PAPERS ............................................................................................................ IX
LIST OF ABBREVATIONS ............................................................................................. X
1.
INTRODUCTION ......................................................................................................1
1.1 GENERAL INTRODUCTION ..............................................................................................1
1.2 CATCHMENT STUDIES ....................................................................................................2
1.3 SOIL AND SOIL SOLUTION ..............................................................................................3
1.3.1 Recharge areas......................................................................................................3
1.3.2 Discharge areas ....................................................................................................4
1.4 STREAMS .......................................................................................................................4
1.4.1 Headwater .............................................................................................................4
1.4.2 Riparian zones.......................................................................................................5
1.5 DISSOLVED ORGANIC CARBON ......................................................................................5
1.6 LOW MOLECULAR MASS ORGANIC ACIDS (LMMOAS) ..................................................7
1.7 WOOD ASH APPLICATION ..............................................................................................8
1.7.1 Wood ash ...............................................................................................................9
1.7.2 Effects ..................................................................................................................11
2. MATERIALS AND METHODS ................................................................................14
2.1 SITE DESCRIPTION .......................................................................................................14
2.1.1 Study site .............................................................................................................14
2.1.2 Streams ................................................................................................................15
2.2 FIELD METHODS ..........................................................................................................15
2.2.1 Stream water .......................................................................................................16
2.2.2 Soil sampling .......................................................................................................16
2.2.3 Lysimeter sampling .............................................................................................16
2.2.4 Centrifugation .....................................................................................................17
2.2.5 Soil respiration ....................................................................................................17
2.2.6 Application of wood ash to the Fanbergs catchment...........................................17
2.3 LABORATORY METHODS .............................................................................................18
2.3.1 Soil and ash .........................................................................................................18
2.3.2 Soil solution and stream water ............................................................................18
2.4 STATISTICAL EVALUATION ..........................................................................................19
vii
3. DEVELOPMENT OF AN ANALYTICAL METHOD FOR ANALYSIS OF
LMMOAS IN NATURAL WATERS .............................................................................21
3.1 SEPARATION................................................................................................................21
3. 2 DETECTION.................................................................................................................22
3. 3 FURTHER DEVELOPMENT ............................................................................................23
4. SOIL, SOIL SOLUTION AND STREAM WATER ................................................25
4.1 SOIL SOLID ..................................................................................................................25
4.2 SOIL RESPIRATION .......................................................................................................26
4.3 SOIL SOLUTION AND STREAM WATER ..........................................................................26
4.3.1 pH and DOC .......................................................................................................27
4.3.2 DOC fractionation...............................................................................................28
4.3.3 Carboxylic content ..............................................................................................29
4.3.4 Low molecular mass organic acids (LMMOAs) ..................................................31
4.3.5 Base cations and their association to DOC.........................................................33
4.3.6 Al and Fe and their association to DOC .............................................................35
5. IN-STREAM VARIATIONS ......................................................................................37
6. WOOD ASH APPLICATION (WAA) ......................................................................38
6.1 The effect of WAA on stream water- and soil solution chemistry in the Fanbergs
catchment .....................................................................................................................38
6.1.1 Respiration ..........................................................................................................38
6.1.2 pH and DOC .......................................................................................................39
6.1.3 Ca and Mg ...........................................................................................................41
6.1.4 K and Na .............................................................................................................43
6.1.5 Transition elements and other metals..................................................................44
6.1.6 Anions..................................................................................................................44
6.1.7 Si..........................................................................................................................45
6.2 IN-STREAM VARIATIONS IN FANBERGSBÄCKEN ...........................................................46
7. CONCLUSIONS AND FURTHER PERSPECTIVES .............................................46
8. ACKNOWLEDGEMENTS .........................................................................................50
9. REFERENCES ...............................................................................................................52
viii
LIST OF PAPERS
This thesis is mainly based on the following six papers, herein referred to by their
Roman numerals:
Paper I
Dan Bylund, Sara H. Norström, Sofia A. Essén, Ulla S. Lundström
(2007) Analysis of low molecular mass organic acids in natural
waters by ion exclusion chromatography tandem mass
spectrometry. Journal of Chromatography A 1176:89-93.
Paper II
Jenny L.K. Vestin, Sara H. Norström, Dan Bylund, Per-Erik
Mellander, Ulla S. Lundström (2008) Soil solution and stream water
chemistry in a forested catchment I: Dynamics. Geoderma 144:256270.
Paper III
Jenny L.K. Vestin, Sara H. Norström, Dan Bylund, Ulla S.
Lundström (2008) Soil solution and stream water chemistry in a
forested catchment II: Influence of organic matter. Geoderma 144:271278.
Paper IV
Sara H. Norström, Jenny L.K. Vestin, Dan Bylund, Ulla S.
Lundström. Influences of dissolved organic carbon on stream water
chemistry in two forested catchments in central Sweden. Accepted for
publication in Biogeochemistry.
Paper V
Sara H. Norström, Dan Bylund, Jenny L.K. Vestin, Ulla S.
Lundström. Initial effects of wood ash application on the water
chemistry of a first order stream in a small, boreal catchment in
central Sweden. Submitted to Water, Air, & Soil Pollution.
Paper VI
Sara H. Norström, Dan Bylund, Jenny L.K. Vestin, Ulla S.
Lundström. Initial effects of wood ash application to soil- and soil
solution chemistry in a small, boreal watershed. Manuscript.
Not included
Sara J.M. Holmström, Sara H. Norström, Anna Rosling. Functional
differences in the genus Piloderma, linking production of LMMOAs
and siderophores to vertical distribution in soil. Submitted to Fungal
Biology.
ix
LIST OF ABBREVIATIONS
AD – Anion deficit
DOC – Dissolved organic carbon
DOM – Dissolved organic matter
ESI – Electrospray ionization
HMM – High molecular mass (Mw>1 kDa)
LC – Liquid chromatography
LMM – Low molecular mass (Mw<1 kDa)
LMMOAs – Low molecular mass organic acids
MS – Mass spectrometry
SIM – Selected ion monitoring
WAA – Wood ash application
WTH – Whole tree harvesting
x
1.
INTRODUCTION
1.1 General introduction
There is no such thing as an isolated piece of nature. Everything in a natural
environment is connected to something else in that environment. Human activities
in one part of such an environment will inevitably cause changes somewhere else,
and our understanding of these relationships is often incomplete.
To achieve a better understanding it is crucial to expand our knowledge of
natural systems and their interfaces in the environments, and how effects travel
through different mediums, especially when proposing large-scale interventions
for these systems.
A variety of influential environmental factors need to be considered when
studying natural systems. Climate, precipitation, elevation, hydrology, topography
and parent material are a few of the more important physical factors, while
fertility, flora and fauna are examples of some biological factors. Each of these has
the potential to influence the environment through interactions with other factors,
creating unique circumstances for a particular area. This makes comparisons of
results from different studies challenging. Solid background data can help
illustrate treatment effects, preferably from the very same spot that the treatment
was administered, despite the fact that chemical properties of different parameters
in soil may show large variations over very small distances. These observations,
when possible, can be combined with vast numbers of other observations from
similar environments, and perhaps general conclusions can be drawn. It is this
methodological collection of field observations that lays the basis for later analysis
and greater understanding.
This thesis addresses the chemistry of soil solution and stream water, two
different mediums that are in constant interaction. Our knowledge of these
interactions is limited; the experiments described in this thesis aim to remedy at
least a small part of that uncertainty in small catchment areas of a boreal forest
environment. In addition to this, the initial effects of wood ash application (WAA)
to the chemistry of these mediums were investigated.
The aims of this thesis were to examine the dynamics of elements and
compounds, the temporal variations in concentrations, and processes determining
the soil properties in a slope within a small, boreal catchment area as well as the
relation between soil and stream water there (papers II, III and IV).
1
The main parameter of study was organic matter and its association of cations
in the catchment. The role of dissolved organic carbon (DOC) as a driver of stream
water chemistry induced by stream water flow was of particular interest, especially
the quantity and quality of DOC, its seasonal fluctuations and its influence on
cation concentrations in the soil and stream water of the investigated area (papers
II, III and IV).
To further investigate the low molecular fraction of DOC, which contains potent
complex-forming substances involved in transport of metals and base cations in
soil and stream water, an improved method for analysis of low molecular mass
organic acids (LMMOAs) was required. Thus a new method employing liquid
chromatography coupled to tandem mass spectrometry (LC-MS/MS) was
developed (paper I) with the aims to provide high selectivity in order to
distinguish between structural isomers, and high sensitivity in order to reach the
low detection limits needed for the analysis of LMMOAs in stream water.
After two years of monitoring the natural variations of the studied
environment, wood ash was applied to one of the investigated catchments. The
aim of this catchment scale intervention was to study the initial effects of this
application on the soil, soil solution and stream water chemistry (papers V and VI).
All results are presented in the form of published articles or manuscripts, and in
the thesis a large summary of the most interesting results is presented together
with some additional unpublished material.
1.2 Catchment studies
Restricted catchments are important for studying and interpreting the processes
in boreal forest ecosystems. This allows short-term fluctuations to be distinguish
from long-term trends and, if possible, to distinguish natural changes from maninduced effects. The nutrient dynamics differ between and within catchments due
to the biogeochemical processes and transport routes depending on the
hydrological conditions (Stutter et al. 2006). This results in different retention and
transformation processes for different substances present in the soil (Johnson et al.
2000).
Stream water reflects the catchments’ hydrological and biogeochemical
conditions since it represents the discharge zones. Chemical analysis of a
headwater stream draining a catchment is a valuable tool for both quantifying the
output and generalising the ongoing processes of a small catchment.
2
1.3 Soil and soil solution
Heterogeneous and complex soil and soil solution systems are the media
through which elements and nutrients are transported to and from plant roots. The
concentrations of elements in these matrices are highly influenced by biological
processes and show large seasonal variations following the activity of biota. To
fairly describe the processes taking place in soils, many measurement replicates
need to be analysed, as some analytes may vary substantially even over short
distances. The temporal variations in concentrations are normally largest in the
upper soil horizons and become smaller with increasing depth in the soil profile
(Johnson et al. 2000; Lundström 1993), although the nutrient concentrations also
vary with soil depth. The soil solution is also the medium for soil-forming
processes, and nutrients such as Na and Si increase in concentration with soil
depth as they mostly originate from weathering reactions. Conversely, K, which is
intensively recirculated in the biota, is found in highest concentrations in the upper
horizons (Johnson et al. 2000). The nutrient concentrations in the soil solution may
therefore reflect both the heterogeneity of the soil, the soil forming processes and
the activity of biota.
1.3.1 Recharge areas
Recharge is the process by which groundwater is replenished. Recharge areas
occupy most of a catchment, and this is where water from precipitation is
transmitted downward in the soil profile through a vertical flow. Soil-forming
processes are continuous in forest soils. In temperate coniferous forests podzols are
the dominating soil type and are generally found in recharge areas. Podzolic soils
are characterized by a thick organic layer (O-horizon or mor layer) composed of
needles, branches, leaves, cones etc. in different stages of degradation. Most of the
organic compounds recovered in soil solution originate from this horizon. Below
the mor layer is a strongly weathered eluvial horizon that is grey to white in colour
(the E-horizon). Here, organic acids (such as LMMOAs, fulvic acids and humic
acids) from the mor layer and exudates of roots and biota weather minerals by
forming complexes with Al and Fe (van Hees & Lundström 2000). The horizon is,
slowly drained of these elements, thereby causing the greyish-white colour. This
soil layer is usually enriched in residual Si. The illuvial horizon (B) is situated
below the E-horizon, and here Al and Fe are immobilized as a result of
precipitation of the organic complexes due to enhanced metal to carbon ratio
and/or microbial degradation of the organic compounds followed by precipitation
of the cations (Lundström et al. 2000a). Al and Fe are accumulated as solid Al-SiOH and Fe-OH phases, giving the horizon its characteristic reddish-brown to dark
3
brown colour (Lundström et al. 2000b). Below the B-horizon is the relatively
undisturbed parent material, also called the C-horizon.
Podzolisation is a slow process and it takes between 300-3000 years for a podzol
profile to fully develop (Lundström et al. 2000a). The thickness of the individual
horizons varies depending on soil age, parent material, vegetation and the climate
at the site (Ilvesniemi et al. 2000).
1.3.2 Discharge areas
Discharge areas are the opposite of recharge areas. They are the locations where
ground water leaves the aquifer and flows to the surface. Discharge areas can
consist of stream channels, dams, riparian zones, irrigation ditches etc., and soils in
discharge areas are usually saturated. The soil profile is subjected to
multidirectional flow patterns. Discharge areas might even be flooded for parts of
the year, leading to anaerobic conditions and slow degradations of organic
substances, and hence, soils in discharge areas are usually enriched in carbon. The
soil-forming processes are thus largely different from those in the recharge area.
No clear vertical division of the soil material is usually visible, in contrast to the
soils of recharge areas.
Water may be stored in the unsaturated soils of catchments during dry periods,
and during high flows the increased ground water level mobilizes this water; the
stream water chemistry varies with flow (Bishop et al. 2004; Kirchner 2003). The
discharge area is important for nutrient dynamics and stream water composition,
and may act either as a sink for the nutrients, as they can be retained by plant
uptake or immobilization, or as a source of nutrients to the stream, especially
during high flows like snow melting (Bishop et al. 1993; Fölster 2000; Laudon et al.
2004). Since stream water chemistry is determined by soil chemistry in the riparian
zone, as this soil is the last contact before the soil solution reaches the stream, the
chemistry in the runoff is essentially a product of flow and riparian soil chemistry
(Bishop et al. 2004).
1.4 Streams
1.4.1 Headwater
A headwater is the part of a stream proximate to its source and a first order
stream is a stream that does not have any other stream feeding into it. The
headwater is therefore the beginning of every first order stream. The chemistry of
stream water in boreal, forested catchments shows large variations during the
4
season with the main driver being precipitation, which in turn affects dissolved
organic matter (DOM) and pH.
In Sweden, a catchment size of 15 km2 is the lower limit for what has been
surveyed on a national scale, but more than 90 % of stream length in Sweden is
actually present in catchments with areas <15 km2 (Bishop et al. 2008). It has been
shown that variability in stream water chemistry for these small catchments may
be in the same order of magnitude as the variability in boreal forest streams in all
of northern Sweden (Temnerud & Bishop 2005), and Ågren et al. (2007) even
proposed that small headwater streams might be the largest contributor to the
terrestrial DOC export per unit area. The importance of, and paucity of knowledge
regarding, headwater has recently been discussed by Bishop et al. (2008) who
stressed the importance of further research on these largely unknown
environments.
1.4.2 Riparian zones
Stream water wetlands, i.e. riparian zones, are strips of wetland areas lining the
stream channel. They link the stream environment to the terrestrial catchment and
act as a buffer between the environments. Substances can be modified, diluted or
concentrated in these zones before they enter the stream (Burt 2005; Luke et al.
2007; Osborne & Kovacic 1993) and riparian areas have in several studies been
shown to be a large source of DOC to stream water (Bishop et al. 2004; Hinton et al.
1998). During high flow events the water table rises in the soil, flow paths change
and old water stored in the unsaturated part of the soil becomes mobilized. Even
superficial flows over the soil may occur. This causes rapid variations in stream
water chemistry as a consequence of increased DOC concentration and changes the
composition of organic matter, and in turn also the amount of cations and nutrients
associated to DOC. Intensive studies of storm events are therefore important as
some of the hydrological processes that influence DOC dynamics occurs only
during these events, and may be missed with a low sampling frequency. In areas
where riparian sources of DOC are important the DOC dynamics will vary
between catchments and may even vary within a catchment during different storm
events (Hinton et al. 1998).
1.5 Dissolved organic carbon
DOC is a ubiquitous substance in soil and natural waters, and plays a crucial
role in establishing the nutrient status of soil as well as in the soil-forming
processes. The organic compounds are derived from degradation of biota and from
roots and mycorrhiza exudates (van Hees et al. 2005). DOC in soil plays an
5
important!role!in!the!biogeochemistry!of!carbon,!nitrogen,!phosphorus!and!metals,!
with!a!corresponding!impact!on!pedogenesis!(Kalbitz!et!al.!2000;!Lundström!et!al.!
2000b).!Due!to!their!complex"forming!ability,!the!organics!affect!the!transportation!
of!different!elements!within!the!soil!(van!Hees!&!Lundström!2000)!and!also!affect!
the!weathering!of!minerals!(Lundström!&!Öhman!1990;!Lundström!1993;!Raulund"
Rasmussen!et!al.!1998).!A!large!proportion!of!cations!may!be!associated!with!DOC,!
which!also!improves!the!moisture!and!structure!of!soil!(Prescott!et!al.!2000).!!
!
Most!DOC!in!streams!originates!from!terrestrial!sources!(allochthonous!carbon),!
where! litter! and! humus! are! the! most! important! contributors! (Kalbitz! et! al.! 2000),!
Definite! compositional! differences! have! been! demonstrated! among! DOC! derived!
from! soil"! and! stream! environments! however,! indicating! that! in"stream!
transformations!take!place!(Malcolm!1990).!DOC!in!stream!ecosystems!is!a!mix!of!
carbon! compounds! produced! by! biota! in! the! stream! (autochthonous! carbon)! and!
allochthonous! carbon! and! holds! great! importance! due! to! its! multi"functional! role!
as!a!primary!food!source!in!the!aquatic!food!web,!in!mobility!and!transport!of!trace!
metals!and!contaminants,!and!in!regulating!acid"base!chemistry!(Hope!et!al.!1994;!
Oliver! et! al.! 1983).! In"stream! processes! are! important! to! DOC! in! stream! water!
(Meyer! et! al.! 1998;! Mulholland! &! Hill! 1997).! For! instance,! leaf! litter! stored! in! a!
stream!channel!may!contribute!to!as!much!as!30!%!of!the!daily!DOC!export!from!a!
stream!in!a!forested!catchment!(Meyer!et!al.!1998).!To!obtain!a!complete!picture!of!
catchment! processes! it! is! important! to! try! to! separate! catchment! hydrological!
processes! from! in"stream! processes.! DOC! in! freshwater! is! also! a! significant!
component!in!the!global!carbon!cycle!(Cole!et!al.!2007).!!
!
DOC! is! commonly! defined! as! the! organic! carbon! derived! from!a!solution! that!
has!been!passed!through!a!0.45!#m"filter,!and!therefore!includes!a!wide!variety!of!
compounds!with!different!sizes!and!properties.!Due!to!its!heterogeneous!nature,!it!
is!hard!to!characterise!without!dividing!it!into!smaller!groups!according!to!certain!
properties,!one!of!which!is!size.!The!low!molecular!mass!(LMM)!fraction!of!DOC!
(<1! kDa)! contains! compounds! such! as! LMMOAs,! peptides,! amino! acids,!
carbohydrates,!siderophores!and!fulvic!acids!(Lundström!et!al.!2000a;!van!Hees!et!
al.!2005)!most!of!which!possess!a!high!ability!to!complex!metals!such!as!Al!and!Fe!!
(van! Hees! &! Lundström! 2000).! The! LMM! compounds! are! derived! mainly! from!
metabolic! pathways! and! are! mineralised! in! soil! within! a! few! days.! The! high!
molecular! mass! (HMM)! fraction! of! DOC! (>1! kDa)! contains! predominantly! humic!
acid!substances!smaller!than!0.45!#m,!which!also!add!to!the!metal"binding!capacity!
of!DOC!(Tipping!1998).!HMM!substances!are!more!recalcitrant!and!may!persist!in!
6!
the environment for years (van Hees et al. 2005). The degradation of DOC is highly
variable due to microbial community, soil structure, climate and aeration.
Elevated DOC concentrations during snowmelt or storms events are a common
feature of nearly all watersheds, indicating that DOC is mobilized by infiltrating
water (Hornberger et al. 1994). Stream water wetlands i.e. riparian zones, are a
large source of DOC to stream water (Bishop et al. 2004; Hinton et al. 1998), and
large parts of Fe and Al in stream water appear to be associated to organic matter
(Pettersson & Bishop 1996).
1.6 Low molecular mass organic acids (LMMOAs)
LMMOAs are versatile molecules defined as aliphatic or aromatic carboxylic
acids with a maximum molecular weight of approximately 300 u. They are
characterized by the possession of at least one carboxylic acid group. Depending
on the dissociation properties and the number of carboxylic groups, these
substances can carry various negative charges and can thereby complex cations in
solutions. LMMOAs contribute to the DOC pool to a small extent comprising a
maximum of 10 % of DOC in soil solution (Strobel 2001; van Hees et al. 2000). They
have a similar vertical distribution as the total DOC content in forest soil, i.e. the
highest concentrations are found in the organic layers and decrease with depth.
LMMOAs are found in all organisms and are involved in, among other things,
energy production in the Krebs cycle and balancing cations in the cells for or
maintaining osmotic potential (Marschner 1995). The major input of LMMOAs to
soil is from root exudations (Sandnes et al. 2005; van Scholl et al. 2006a), forest
litter (dead plant materials and microbial decomposition products; Pohlman &
McColl 1988), and ectomycorrhiza in symbiotic relationship with tree roots
(Ahonen-Jonnarth et al. 2000; van Hees et al. 2006; van Scholl et al. 2006a; van
Scholl et al. 2006b). The LMMOAs are implicated in many soil processes including
mobilization and uptake of nutrients (Fransson et al. 2004; van Breemen et al.
2000), dissolution of soil minerals leading to pedogenesis (Lundström et al. 2000b),
and microbial proliferation in the rhizosphere (Marschner 1995). They are
generally found in higher concentration in rhizosphere soil than in bulk soil (Jones
1998). LMMOAs are also produced by photochemical transformation/degradation
by DOC in freshwaters (Bertilsson & Tranvik 2000).
The ability of LMMOAs to complex metals and affect soil pH has been well
investigated (Lundström & Öhman 1990; van Hees & Lundström 2000; van Hees et
al. 2001). Organic acids with only one carboxyl group (e.g. lactate, formate, acetate)
7
have low metal-complexing abilities. The degree of complex-forming depends on
the organic acid in question (number and proximity of carboxyl groups), the type
of metal and its concentration, and the pH of the solution. Di- and tri-carboxylic
acids can be potent metal complex-formers acting as detoxifiers of Al in soil
solution (Hue et al. 1986) and bringing about the dissolution of soil minerals and
modulating the size of humic substances (Jones 1998). LMMOAs are also known to
increase in different parts of plants i.e. roots, leaves and xylem sap, as a response to
Fe deficiency as reviewed by (Abadia et al. 2002), and species induced weathering
has been related to the production of individual LMMOAs in the soil (Holmström
et al. submitted).
Organic acids have high turnover rates in soil, with half-lives ranging from 1-5
h in top-soil and approximately 12 h in subsoil (Jones 1998; van Hees et al. 2002).
Basic calculations using typical soil solution concentrations of LMMOAs, amino
acids and simple sugars indicates that their contribution to respiration could be as
high as 30 % (van Hees et al. 2005). However, concerns have been raised that
current methods of quantifying organic acids may vastly underestimate the
concentrations in soil solution, due to the high degree of buffering by the soil solid
phase, the continual removal by the microbial community and the fact that
available sampling techniques do not take into account the spatial heterogeneity of
these substances (Jones et al. 2003). LMMOAs have rarely been identified or
quantified in stream water, a medium that due to its close proximity to soil may
offer useful insight into the behavior of these substances. A wide variety of
techniques have previously been used for analyzing LMMOAs in soil solution, e.g.
reversed phase liquid chromatography, ion chromatography, gas chromatography
and capillary zone electrophoresis (Hagberg 2003; van Hees et al. 1999).
1.7 Wood ash application
Whole tree harvesting (WTH) in Swedish forestry has increased in recent years
due to the novel interest in renewable energy sources. Previously only stem wood
was recovered at harvest, while branches, tips and needles/foliage - parts that
contains the most amounts of mineral nutrients were left to decompose at the
harvest site, contributing to the recycling of mineral nutrients to the forest soils.
WTH has been shown to significantly decrease cation exchange capacity and total
amounts of exchangeable base cations compared with conventional stem harvest in
the uppermost soil layers 15 years after harvest (Olsson et al. 1996) and to increase
soil acidity and reduce base saturation 28 years after harvest (Vanguelova et al.
2010). It has also been shown that leaving slash after forest harvest has a positive
effect on nutrient status in foliage for successive forest stands (Olsson et al. 2000)
8
and that WTH can have a negative effect on tree growth already in the second
rotation (Proe et al. 1996; Walmsley et al. 2009).
Increased use of forestry products as bio-energy also gives rise to by-products
in the form of wood ash. Wood ash is a highly alkaline substance that up until
recently has been dumped in landfills. To address the concerns that depletion of
nutrients in forest soil will arise with the more extended use of WTH, the Swedish
Forest Agency now recommends WAA to recycle the nutrients in these forests. The
main aim of WAA is to counteract acidification in soil and runoff from forests
subjected to WTH. A positive side effect is an alternative disposal route for byproducts from the forest industry. It is also conceivable that increased growth rates
in the treated forests may result. In Sweden a maximum amount of 6 ton/ha per
rotation is to be applied to areas subjected to WTH, and not more than 3 ton/ha per
10-year period. WAA is recommended at least once in every rotation (Anonymous
2008).
When wading through the literature discussing the effect of wood ash
application to forest ecosystems the most striking feature is the abundance of
variables that can be changed in an investigation. Ash type is a typical example
variable; different wood products and incineration ovens can be used resulting in
markedly varying ash quality. Similarly, treatments of the ash can encompass any
of the following: selfhardening, granulation or pelletzing. Additives may include
any combination of dolomite, lime, N- and P-fertilizers. The quantity and
frequency of application is also highly variable, as is the season of distribution.
Geographical features such as clearcuts and mires, as well as fertility status in the
forest (Saarsalmi et al. 2001a) are additional variables. Investigation parameters
such as the time after application until investigation, timing and presence of
control plot establishment, number of replicates used, choice of medium sampled
and method used to do so, choice of analytes for analysis and method used to do
so etc. all make it difficult to draw general conclusions from the literature.
1.7.1 Wood ash
The main types of crystalline compounds formed during combustion of forest
products, such as sawmill residues, are oxides, carbonates, sulphates and
chlorides. Nitrogen compounds are lost in fuel gases when wood is combusted.
Reactive oxides and soluble salts are compounds that can cause undesirable effects
in forest biotopes such as pH shock, burning damages to plant tissues and salt
effects (Steenari & Lindqvist 1997). To avoid this, the ash must be stabilized.
Stabilisation can be achieved by pelletizing, granulation or self-hardening. The self9
hardening!process!utilizes!the!ability!of!most!ash!materials!to!solidify!on!addition!
of! water.! Water! is! also! added! in! the! pelletizing! and! granulation! technique! but!
instead!a!mechanical!process!is!employed,!while!self"hardening!ash!is!usually!left!
out!in!the!open!to!solidify!after!which!it!is!crushed.!!
!
During! the! self"hardening! process! the!dominating! reactions! in! the! presence! of!
CO2!are:!
!
!"# $ %& # ' !"(#%)& !
!"(#%)& $ !#& ' !"!#* $ %& #!
!
These! reactions! occur! in! consecutive! steps.! The! hydration! of! CaO! is! rapid! and!
exothermic,! then! the! CO2! dissolved! in! the! pore! fluid! of! the! hydrated! ash! is!
transported!by!diffusion!into!the!material!where!crystalline!CaCO3!is!formed.!The!
carbonation! of! Ca(OH)2! (portlandite)! is! hence! dependent! on! the! presence! of! a!
water!phase!in!which!the!reactants!can!be!dissolved!and!transported.!The!calcite,!
CaCO3,!that!forms!precipitates!from!the!solution!and!creates!a!product!layer!on!the!
ash!surface!and!in!the!pores.!Both!the!hydroxide!and!carbonate!formation!lead!to!a!
solidification! of! the! wetted! ash.! During! the! hardening! process,! the! ash! surface!
layer! gradually! becomes! hard! and! brittle! as! the! crystallization! of! CaCO3! proceed!
(Steenari!&!Lindqvist!1997;!Steenari!et!al.!1998).!
!
Ashes!with!high!content!of!unburnt!matter!do!not!harden!on!addition!of!water,!
and! the! degree! of! hardening! decides! the! reactivity! of! the! ash.! Calcite! is!
approximately! a! hundred! times! less! soluble! than! calcium! oxide! or! calcium!
hydroxide,! which! is! why! carbonatisation! of! the! ash! is! an! important! part! of! the!
stabilization.!The! formation! of! a! hard,!dense! structure! generally!leads! to! reduced!
leaching!rates!for!most!nutrients!(Steenari!&!Lindqvist!1997).!
!
Since! carbonatisaton! primarily! occurs! in! the! surface! layer! of! the! ash! pile,! the!
reaction!in!the!interior!part!is!slow.!This!can!be!due!to!the!formation!of!a!product!
layer!of!calcite!hindering!the!transport!of!carbon!dioxide!to!the!interior!of!the!pile,!
or! a! drying! of! the! surface! layer! though! the! reaction! takes! place! in! solution.! To!
speed!up!the!carbonation!process!the!ash!may!be!turned!over!and!wetted.!As!the!
ash!has!been!spread!in!the!forest,!the!ash!particles!will!become!more!available!for!
carbonatisation! and! this! process! may! continue! (Steenari! et! al.! 1999).! In! a!
comparison!between!untreated!ash!and!ash!that!had!been!stored!outdoors!or!in!the!
laboratory! it! was! found! that! the! leaching! of! Ca! significantly! decreased! by! the!
carbonation!reaction.!!
10!
The amount of Ca released from the ash is highly dependent on the form it is
present in. If the ash is well stabilized, Ca is present as carbonate while a less
stabilised ash will have Ca in the form of hydroxides and oxides (Steenari et al.
1998). The dissolution of Ca species at the ash surface, i.e. in the pore fluid, is pH
dependent. One example given is Ca(OH)2, whose solubility is very low at a pH
above 12.5 but rapidly increases with decreasing pH owing to CO2 equilibration
and loss of alkali metal hydroxides by leaching.
The self-hardening processes increased the leaching of K and Na, probably due
to a decrease in particle size. A larger surface area is thus exposed to the leachate
and this facilitating the dissolution and transport of these substances (Steenari et al.
1999). In a column leaching experiment, Ca, K, Na Cl and SO4 leached rapidly,
while Mg and P seem to be released more slowly from the ash (Eriksson 1998b).
Similar observations were made by Steenari et al. (1999) who also found low
leaching rates for Fe, Mn, Zn, Cu, Si and Pb, Cd and very low Hg levels in
leachates.
1.7.2 Effects
A number of studies have investigated WAA in forests and its subsequent
effects on soil processes (Lundström et al. 2003a), forests and aquatic systems
(Aronsson & Ekelund 2004), temperate forest ecosystems (Augusto et al. 2008) and
environmental impacts in forestry (Pitman 2006). In most of the investigations, soil
samples and the exchangeable pool of nutrients have been studied, while fewer
studies addressed the effects on soil solution chemistry and stream water
chemistry.
Application of wood ash has different effects depending on soil type and
nutrient status in the soil. For instance, addition of stabilized wood ash to
coniferous stands on mineral soil did not significantly affect stem growth (Jacobson
2003). Increased growth occurred when N was added however, alone or in
combination with wood ash, and there were indications that WAA would increase
stem growth at nutrient rich sites while conversely decreasing growth in less fertile
sites (Jacobson 2003). Högbom et al. (2001) found that WAA may increase leaching
of NO3- from sites with relatively high N-input.
Wood ash is sometimes referred to as fertilization, and indeed increased growth
rate of soil microorganisms and increased respiration at the soil surface has been
observed (Zimmermann & Frey 2002). Perkiömäki & Fritze (2002) found that WAA
onto the forest floor of coniferous forests caused a dose-dependent increase in
11
microbial activity and a change in the community structure in the mor layer
independent of the fertility of the site. Other groups have had more difficulty in
confirming this. Maljanen et al. (2006a) found no initial change in the respiration
rate due to WAA following application of 7 ton/ha of loose wood ash to a forested
site in southern Finland. However, when investigating WAA (3-8 ton/ha) in
northern Finland on both mineral- and peat soils increased respiration rate on old
ash plots (14-50 years) was observed, but no increase in newly established
experiments (1 year; Maljanen et al. 2006b). Rosenberg et al. (2010) found increased
respiration 12 years after application of 6 ton/ha to a Norway spruce site in
southern Sweden and Taylor & Finlay (2003) found changes in the mycorrhizal
community in a Norway spruce forest 4 years after application of 4.28 ton/ha of
wood ash. Jokinen et al. (2006) tried to distinguish what change, increased pH or
DOC, caused by the WAA was responsible for the increase in microbial activity
and demonstrated that both pH and DOC quality influenced subsets of the
bacterial community.
When applying ash to forests the aim is generally to raise the pH of acidic soils
and surface waters, and to improve the nutrient status of the soil. Increases in
cation exchange capacity (CEC), base saturation (BS), pH and decreases in total
acidity (TA) in the mor layer have been found in several studies after application of
3 ton DS/ha of wood ash to forest stands of different ages in Sweden and Finland
(Arvidsson & Lundkvist 2003; Eriksson 1998a; Saarsalmi et al. 2001b).
In investigations of the effects of WAA on soil solution chemistry, Geibe et al.
(2003) found increased concentration of Ca, Mg and DOC in soil solution 4 years
after application of 4.28 ton TS/ha and in the same investigation Holmström et al.
(2003) found that the increase in DOC was attributed to an increase in the HMM
fraction, in turn leading to a more hydrophobic constitution of DOC. Ring et al.
(2006) observed increases in K, Na, SO4 and TOC in soil solution sampled at 50 cm
depth after application of 3 ton/ha crushed wood ash.
In investigations of the effects of WAA (2.4 ton/ha) to the soil solution
chemistry in a pine stand in northern Germany, the DOC concentration did not
increase. An initial decrease of pH due to desorption of exchangeable cations when
soluble salts were leached from the ash or desorption of Al followed by
precipitation of Al(OH)3 was noted, however. Additionally, elevated
concentrations of Ca, K, SO4 and NO3 were found furthermost in the topsoil, but
some changes were visible down to 100 cm (Ludwig et al. 2002; Rumpf et al. 2001).
12
Information regarding the effects of wood ash application on the chemistry of
streaming water is scarce, with a few exceptions. A study by Tulonen et al. (2002),
for instance, reported that WAA (6.4 ton/ha) to 12 % of a catchment area drained
by two small streams to a humic lake led to a slight increase of conductivity,
alkalinity, K, Cl, and SO4 concentrations for the stream running through peat soil
and pH, K and Cl in stream water running through mineral soil. The increases
were only significant for K in the peat soil stream however, and for pH and K in
the mineral soil the first year after ash application. Changes in the same parameters
were also observed in the recipient lake. In another study, three adjacent
catchments of 20 ha were treated with 2.2 ton/ha granulated wood fly ash, 5 ton/ha
dolomitic limestone or remained untreated as a reference respectively (Fransman &
Nihlgård 1995). Compared to the reference, the ash treatment resulted in increased
K concentrations and a tendency towards increased pH five years after treatment.
In the limed catchment stream, pH, Ca and Mg increased and Al, Fe and Mn
decreased over the same period.
Studies of stream water draining lime-treated catchments are more plentiful.
Löfgren et al. (2009) studied nine stream waters draining catchments treated with 3
ton/ha of lime or dolomite and ten untreated stream waters in southern Sweden.
No significant differences were observed between treated and untreated stream
waters 12-16 years after treatment. Regardless of treatment, however, all stream
waters exhibited a general pattern of declining concentrations of SO4, Ca, BS and
increasing acid neutralizing capacity (ANC), likely due to the reduced sulphur
deposition. In contrast, Hindar et al. (2003) investigated a stream water draining a
84 ha catchment treated with 3 ton/ha of dolomite and reported significant
increases in pH, Ca, Mg, ANC and TOC and decrease of inorganic monomeric Al
over a 6-year period after liming compared to a reference stream.
Aronsson & Ekelund (2008) investigated Fanbergsbäcken, the same stream
studied in this thesis, 2 years after WAA and found no changes in bentic diatom
community, but indices of elevated concentrations of K in aquatic moss and leaves
from alder. No heavy metal accumulation was detected by either Moilanen et al.
(2006) or Levula et al. (2000) in berries or mushrooms in different time intervals
after subjection to different doses of wood ash. In contrast, the concentrations were
often lower than those in the controls (except for As) in older experiments (13-52
years; Moilanen et al. 2006).
13
2. MATERIALS AND METHODS
Figure 1. The Gråbergs catchment (control) and Fanbergs catchment to the left. The
numbers 1-3 denotes the sampling points in each stream. Area for intensive soil sampling
in the Fanbergs catchment enlarged to the right. White circles denotes lysimeter sampling
pits in the discharge and recharge area.
2.1 Site description
2.1.1 Study site
The field studies (papers II, III, IV, V and VI) took place in two small, bordering
forested catchments, 50 and 40 ha respectively, in Bispgården (63 07N, 16 70E),
central Sweden (Figure 1). Each catchment is drained by a first order forest stream.
The forest consists mainly of 50 to 80 year-old Norway spruce (Picea abies) and
Scots pine (Pinus sylvestris). The catchments are located at an altitude of 258 m
above sea level, which is above the highest coastline in this area, so postglacial
surficial sediments that are common to postglacial deltas are lacking. The area has
only low levels of anthropogenic deposition. The bedrock is mainly composed of
granite and gneiss. The soil in the recharge area is classified as podzol and in the
discharge area the soil is classified as arenosol (FAO, 1990).
The investigation in this thesis involved six plots at 10 m from the stream
(arenosol) and six plots at 80 m from the stream (podzol). The plots were separated
by 10-15 m each (Figure 1). The organic horizon was on average 28 cm thick in both
podzol and arenosol. The podzol had an average depth of 9 cm for E-horizons and
7 cm for the B-horizons above the C-horizons. The arenosol had an average depth
14
of 50 cm for the A-horizons above the C-horizons. When sampling the mor layer,
the living vegetation was removed and the remaining mor was divided into two
parts. The upper part (O1) consisted of partly degraded litter and the lower part
(O2) consisted of well-degraded litter.
2.1.2 Streams
The location and distribution of wetlands and the morphology of the streams
differed between the two catchments. Both streams, Fanbergbäcken and
Gråbergsbäcken, originate from small mires, with estimated areas of about 0.2 ha
and 1 ha, respectively. Gråbergsbäcken, however, also receives water from a
spring, making it less sensitive to drought. Like the majority of the forest streams
in central and northern Sweden, these streams have been subjected to ditching,
which is common in many peatlands and wet mineral soils to improve forest
productivity (Ivarsson & Jansson 1994). The scars from this treatment are more
visible in Gråbergsbäcken, where the streambed cuts deeper into the soil and the
stream water lacks contact with the shallow organic soil layers for approximately
80 % of its length. An increase in flow would therefore raise the surface of
Gråbergsbäcken, but essentially no flooding of the banks would occur. The deeper
streambed might also lead to a greater influence of groundwater from deeper flow
paths in the soil. In Fanbergsbäcken, the traces of the ditching are less obvious, and
the stream is shallower with a more pronounced riparian zone that lines at least 60
% of the stream and is evenly distributed throughout its length. This would cause
the riverbed to expand up over the banks in several areas along the stream at high
flows. Even though the areas of wetlands are similar between the catchment their
location differs, with wetlands lining Fanbergsbäcken as a pronounced riparian
zone, while located apart from the stream in the Gråbergs catchment.
2.2 Field methods
The sampling seasons lasted from April to November in 2003-2006 as the soil
and streams were frozen in winter. The schedule of sampling methods is listed in
Table 1.
15
Table 1. Schematic description of the water sampling conducted. C - control, A - wood ash
applied.
2003
2004
2005
2006
2007
Soil solution centrifugation
C
Soil solution lysimeter
C
Ca
A/Cb
A/Cb
a
Stream water Fanbergsbäcken
C
C
A
A
Ac
Stream water Gråbergsbäcken
C
C
C
C
C
aAsh was applied to the catchment in late September 2004
bA control area without ash saved within the catchment (see Figure 1)
cForest was harvested in the Fanberg catchment possibly causing an interfering effect
2.2.1 Stream water
Stream water samples were collected manually every second week and with
higher frequency during high flows. The sampling points were located at the outlet
of each catchment and at two additional sites upstream (Fan1, Fan2, Fan3, Grå1
Grå2, Grå3; Figure 1). The water was collected in polyethylene bottles and kept
cool until pre-treatment within 24 h after sampling.
2.2.2 Soil sampling
In 2002, soil samples for determining soil pH and soil, exchangeable cations,
and total content of nitrogen and carbon were collected from all horizons in all
pits. On the same occasion soil samples for determination of total composition
were taken from two plots in podzol and two plots in arenosol including all
horizons except for the mor layer (E, B1, C in podzol; 0-5 cm, 10-15 cm, 45-50 cm in
arenosol). Additional soil samples for determination of exchangeable cations, and
pH in centrifugated soil were collected from all horizons in all pits in 2004 and
2007.
2.2.3 Lysimeter sampling
Suction lysimeters (Rhizon MOM, Rhizosphere, Wageningen, Netherlands)
were installed at four horizons: O2 (lower mor layer), E, B1 (upper B-horizon) and
C in podzol soil and O2, 0-5 cm, 10-15 cm and 45-50 cm depth in arenosol. Three
lysimeters were installed at each horizon in each pit. Lysimeters were used to
sample the percolating soil solution that might influence stream water
composition. After sampling, the soil solutions from the triplicate lysimeters were
mixed to obtain a composite sample from each horizon and pit. Soil water from the
lysimeters was collected every month during the sampling season.
16
2.2.4 Centrifugation
In June 2004 soil was collected to obtain soil solution samples by centrifugation
according to Giesler & Lundström (1993). The soil samples were taken from O1,
O2, E, B1 and C-horizons in podzol and from O1, O2, 0-5 cm, 10-15 cm and 45-50
cm depths in arenosol. After centrifugation, the soil solution was filtrated through
0.45 µm membrane filters (Millex-HV, Millipore) before analyses.
2.2.5 Soil respiration
The soil respiration was measured every month during the growing seasons at
two pits in the recharge area and at two pits in the discharge area. The
measurements were made at eight points around each pit and an average value
was calculated. The respiration was measured using an EGM-4 (PP-Systems, UK)
equipped with a soil respiration chamber (SRC-1) and a soil temperature probe
(STP-1).
2.2.6 Application of wood ash to the Fanbergs catchment
In late September 2004 self-hardened crushed wood ash was spread in the
Fanbergs catchment, at a dosage of 3 ton DS/ha as recommended by the Swedish
Forest Agency (Anonymous 2008; Figure 1). No ash was spread in the control area
and a 10 m buffer zone surrounding it (paper VI). The results from previous
studies conducted in the same area (papers II, III and IV) were used as background
data. The stream draining the ashed catchment, Fanbergsbäcken, was intensively
monitored, and the stream draining the bordering catchment, Gråbergsbäcken, was
used as an untreated reference.
The elemental composition of the wood ash, the recommended minimum
amounts for some of the elements, and the composition of the top mineral soil
close to Fanbergsbäcken are described in Table 2. The wood ash met the Swedish
Forest Agency standards for the elements investigated in this study (Anonymous
2008).
17
Table 2. Composition (g/kg DS) of the wood ash, recommended minimum amounts from the
Swedish Forest Agency (Min; Anonymous 2008), and the total composition found in the
superficial mineral soil close to the stream (Arenosol 0-5).
Arenosol
Element
Ash
Min
0-5 cm
Si
97.2
238.9
Al
29.7
42.8
Ca
205.1
>125
6.6
Fe
13.0
10.9
K
30.6
>30
18.7
Mg
15.7
>15
1.8
Mn
12.4
0.2
Na
6.2
12.3
2.3 Laboratory methods
2.3.1 Soil and ash
Soil samples for determination of total composition were dried at 105 °C,
melted with LiBO4 and dissolved in HNO3 before analysis by inductively coupled
plasma mass spectrometry (ICP-MS). The analyses were performed at Analytica in
Luleå, Sweden. Samples for N and C content were dried at 70°C before analysed
with a Perkin Elmer CHN 2400 elemental analyzer. These analyses were made at
the Soil Science Laboratory at the Swedish University of Agricultural Sciences
(SLU) in Umeå, Sweden. Determination of exchangeable cations, soil content of N
and C and pH was conducted twice, first in 2002 at HSMiljölab AB in Kalmar,
Sweden (paper II), and for centrifugated soil from 2004 and 2007 at Agrilab AB in
Uppsala, Sweden (paper VI). The total composition of the ash was determined by
melting with LiBO2 followed by dissolution in diluted HNO3. For determination of
As, Cd, Co, Cu, Hg, Ni, Pb, S and Zn the sample was dried at 50°C and dissolved
in HNO3/HCl/HF in a closed Teflon container in a microwave oven. The samples
were then analyzed with a combination of ICP-AES and ICP-SFMS. The analyses
were performed at Analytica.
2.3.2 Soil solution and stream water
All samples were stored refrigerated until analysis unless otherwise mentioned.
The pH was determined within 24 h using a Beckman Φ32 pH-meter. For all other
analyses the stream water was filtrated through a 0.45 µm membrane filter (MillexHV, Millipore). Samples for cation analysis (Ca, Mg, Fe, Al, K, Na, Si, Cr, Mn, Co,
18
Ni, Cu, Zn, Cd, Pb) were acidified with ultrapure HNO3 before analysis with ICPMS (VG PQ ExCell, Thermoelemental, Winsford, England). DOC was determined
within 48 h of sampling and quantification was achieved by catalytic oxidation and
IR detection with a Shimadzu TOC analyzer, TOC-5050A (Shimadzu, Kyoto,
Japan). Anion determination of F, Cl and SO4 was performed within 48 h of
sampling by ion chromatography with Dionex DX-120 (Dionex, Sunnyvale, CA,
USA). Flow injection analysis (FIA) with a FIAstar 5000 (Foss, Höganäs, Sweden)
was used for the determination of quickly reactive aluminium (Alqr; Clarke et al.
1992) and NO3 within 48 h of sampling. Samples for analysis of NH4 and PO4 were
stored frozen until analysis with FIA.
Some stream water samples were separated into a smaller size fraction
(nominal molecular weight cutoff 1 kDa), hereinafter referred to as the LMM
fraction of the substance, by means of a stirred cell method (Amicon model 8050),
in which the water samples are passed through a YM-1 membrane of regenerated
cellulose (Millipore Corporation, Billerica, MA 01821) under a N2 pressure of 3.5
bar. For further details on this method see van Hees et al. (2001). DOC and cations
were determined for these samples and the amount of elements associated with the
high molecular mass (HMM) fraction (Mw>1 kDa) was calculated by subtracting
the concentration recovered in the LMM-fraction (Mw<1 kDa) from the total
concentration.
Samples for determination of total acidity were stored frozen until analysis by
titration. After thawing, the sample was first passed through a column with a
strong cation exchange resin (Dowex 50 8), and then titrated with 0.01 M NaOH
to pH 7. A constant flow of N2 was supplied to the sample to remove any
atmospheric CO2 during the titration, which was performed with an autoburette
system (702 SM Titrino, Metrohm Ltd., Herisau, Switzerland) and a glass electrode
(Mettler-Toledo). For further details on this method, see Hruska et al. (1996).
Samples for analysis of LMMOAs were stored frozen and then determined by
LC-MS/MS (paper I). The LC-MS/MS system was comprised of a Shimadzu LC10AD pump, an Agilent 1100 autoinjector, a Supelcogel C610-H column (300 7.8
mm) and an API3000 mass spectrometer (MDS Sciex, Concord, Canada). The
mobile phase consisted of 10 % (v/v) methanol and 0.01 % (v/v) formic acid in
MilliQ-water.
2.4 Statistical evaluation
Microsoft Office Excel 2007 (Microsoft, Redmond, WA), MINITAB version 12
(Minitab Inc., State College, PA), MATLAB 6.1 (The MathWorks Inc., Natick, MA)
19
and Sigmaplot 11 (Systat Software Inc., San Jose, CA) were used for mathematical
treatment and statistical evaluation of data. Unless otherwise stated, the
significance level was set to 0.05, i.e. an observed effect was considered as
significant and the null hypothesis of no effect rejected if it was indicated that its
probability to be valid was below 5%.
Correlations between time series were evaluated by linear regression followed
by t-tests to evaluate the significance of the Pearson correlation coefficients
obtained (paper IV). Student’s t-test was in general used to determine the
significance of differences in the means between two data series (papers IV, V, VI),
and whenever appropriate, paired t-tests were used in order to minimise the
influence of temporal variations in the data. When more than two means or
underlying causes of variations were to be evaluated, analysis of variance
(ANOVA) was applied (paper II).
In some cases, it could be doubted whether the requirement of naturally
distributed data for the above mentioned tests was fulfilled. In paper IV, a robust
sign test (Miller & Miller 2000) was thus used to evaluate differences in the size
fractionation of different cations expressed in percentage, and in paper V, a
random intervention analysis (RIA; Carpenter et al. 1989) was used as a
complement to t-tests when evaluating the impact of WAA to stream water
chemistry. In RIA, random permutations of the test variable are performed in order
to establish a distribution of the treatment effect, which enables determination of
the likelihood of obtaining the experimental value. In this particular case, Monte
Carlo simulations with 10 000 permutations were performed for each analyte to
establish the p-values for the measured effects.
Finally, cumulative sum (cusum) charts (Figure 2) was used to visualise small
changes within time series and to evaluate whether observed effects are persistent
or due to more abrupt, short-lived changes in the water chemistry (paper V).
20
Figure 2. Seasonal variation of stream water concentration of Ca in Fanbergsbäcken (Fan)
and Gråbergsbäcken (Grå) in 2003-2006 (left) and the concentration of Ca in
Gråbergsbäcken subtracted from Fanbergsbäcken and the cumulative difference in Ca
(right).
3. DEVELOPMENT OF AN ANALYTICAL METHOD FOR
ANALYSIS OF LMMOAS IN NATURAL WATERS
LMMOAs have frequently been analyzed in soil solution, where they are
present in relatively high concentrations and may constitute of up to 10 % of the
total DOC in soil solution (Strobel 2001; van Hees et al. 2000). Comparatively few
investigations of stream water have been conducted, as this water is usually more
diluted, contains less organic matter and requires a more sensitive analytical
method. Of the acids investigated in the current study, only the levels of lactic acid
and citric acid had previously been reported in surface waters (Hlavacova et al.
2005; Jonsson et al. 2007). A robust and selective method with low detection limits
for LMMOAs present in natural waters was therefore developed as part of the
research work described in this thesis. The new method coupled HPLC on-line, via
an electrospray ionization interface, to mass spectrometry detection.
3.1 Separation
A number of other methods have previously been applied in the separation of
LMMOAs in natural waters. Most common was the use of ion exclusion
chromatography (van Hees et al. 1999) and capillary electrophoresis (Hagberg
2003; Hagberg et al. 2002). Since ion exclusion chromatography is well compatible
with MS-detection, the original method by van Hees et al. (1999) was here selected
for further development.
21
Ion exclusion chromatography of acidic compounds uses a negatively charged
stationary phase, typically partly sulfonated divinyl benzene, which repels anions
due to so-called Donnan exclusion. Compounds, exhibiting a negative charge
under the chromatographic conditions applied, will elute in the void volume of the
column. Nonionic or weakly ionic substances, on the other hand, will be retained
as they can penetrate into the pores of the packing material. Thus, in pure ion
exclusion chromatography, the retention time of a solute is solely dependent on its
net charge. Size exclusion effects and secondary interactions with the stationary
phase such as van der Waal and hydrophobic interactions will, however, also
influence the separation to various degrees and add to the selectivity of the system.
Thus the LMMOAs will separate on the ion exclusion column primarily in order of
their pKa values, and secondarily according to size and other properties of the
analyte.
3. 2 Detection
Low UV-range spectrometry (~210 nm) has been used to detect aliphatic acids,
which essentially lack strong chromophores (Hees et al. 1999). In this UV-range a
large number of organic substances absorb light, making peak separation difficult
and peak identification uncertain due to interference and coelution. An alternative
is to apply indirect UV detection as described by Hagberg (2003), but this mode of
operation also suffers from selectivity problems. In the work described in this
thesis, electrospray ionization (ESI) in combination with a tandem mass
spectrometer (MS/MS) improved the overall selectivity and attained the low
detection limit needed for the applications intended. In electrospray, the liquid
mobile phase gets converted into a fine arenosol of charged droplets that dries
until a critical volume where a coulombic explosion occurs, a process finally
resulting in the production of gas phase ions from the analyte. The ions then enter
the MS where their mass-to-charge ratios (m/z) and abundances are measured
(SIM mode). Alternatively, the ions can be accelerated into a collision cell, where
collisions with inert gas molecules results in fragmentation producing a unique
mass pattern for detection.
A number of adjustments were made to modify the original method based on
HPLC-UV developed by van Hees et al. (1999) to allow proper function with MS
detection. The eluent was adapted to the ESI-interface by exchanging the
phosphoric acid with formic acid (0.01 % v/v) for its higher volatility, and by
adding 10 % methanol the hydrophobic interaction with the stationary phase was
lowered and a more stable electrospray was obtained. A multiple reaction
22
monitoring mode was used for detection and the 17 acids included in the method
were separated within 30 minutes (Figure 3).
Figure 3. A 5 µM standard of 17 different LMMOAs analyzed with LC-ESI-MS/MS
3. 3 Further development
The chromatographic separation for the structural isomers, fumarate and
maleat and trans- and cis-aconitate, was excellent (Figure 3). Citrate and isocitrate
may, however, due to their similar retention time constitute a problem while
integrating, especially if large differences in concentrations are present. A high
signal from citrate may shield or in the worst case completely swallow a low signal
from isocitrate. To circumvent this, an additional mass channel, unique to
isocitrate, has been added to the method. Figure 4 shows the product ion spectrum
for citrate with the mother ion m/z=191 and the main daughter ion at m/z=111
representing neutral losses of two H2O and one CO2.
23
Figure 4. ESI-MS/MS spectrum for citrate after collision induced fragmentation at Elab=18
eV.
The same mass channel was used for isocitrate in the original method (paper I).
Isocitrate, however, has several more usable fragments that are not formed in the
fragmentation of citrate, and as a complement the ion with m/z=117 was chosen
(Figure 5). This mass channel does not possess the same sensitivity as the original,
but it contributes to a more definite identification of the isocitrate peak, and may
also be used for quantification in difficult samples.
24
Figure 5. ESI-MS/MS spectrum for isocitrate after collision induced fragmentation at
Elab=23eV.
4. SOIL, SOIL SOLUTION AND STREAM WATER
The result from the pre-ash application study, based on the data sets marked with
a C in table 1 are discussed in this section.
4.1 Soil solid
The parent material had a similar composition throughout the investigated
slope in the catchment. A trend towards higher carbon content was found
throughout the soil profile in the arenosol compared to podzol with the exception
of the O horizon (paper II). The higher carbon content in the discharge areas was
due to less oxidizing conditions and a slower decomposition rate induced by a
wetter environment. The content of exchangeable Ca was generally higher in
arenosol throughout the soil profile, and the exchangeable Mg, K and Na contents
were higher in superficial layers of the arenosol compared to podzol. This resulted
in a higher base saturation and a trend towards a higher content of exchangeable
25
cations in all mineral horizons in arenosol, probably an effect of the higher carbon
content (paper II).
4.2 Soil respiration
Soil respiration measurements did not show any significant difference between
the recharge and discharge area and varied in the range of ~1-10 µmol CO2/m2s
over the 2003-2004 sampling season (Figure 12). The activity of biota in the
superficial soil was therefore assumed to be similar for podzol and arenosol. Any
difference in soil solution most likely reflected soil properties and soil water
content rather than biotic activity.
4.3 Soil solution and stream water
The concentration in the E- and O-horizon of most of the measured substances
peaked in late summer due to build up of products of weathering and degradation
of biota that were subsequently washed out during autumn rains. The
concentration of many of the measured substances differed between the recharge
and the discharge area (Table 3). As seen in Table 3, most of the analyzed
substances are higher in the discharge area, which is in agreement with Johnson et
al. (2000) who found that both soil and drainage waters showed a consistent
pattern of nutrient enrichment downslope when investigating biogeochemical
processes in small catchments.
Table 3. Significant differences in seasonal averages for soil solution from lysimeter
sampling 2003-2004. P= significantly higher in podzol (recharge area), A= significantly
higher in arenosol (discharge area) (p≤0.05).
pH
DOC
NO3-
F-
Cl-
SO42-
Na
Mg
Al
Si
P
P
K
Ca
Mn
Fe
2003
O
E 0-5
P
A
B1 10-15
P
A
A
C 45-50
P
A
A
A
A
A
A
A
A
P
P
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
2004
O
E 0-5
P
A
B1 10-15
P
A
A
A
C 45-50
P
A
A
A
A
A
26
A
A
A
A
A
A
P
A
A
A
A
A
A
A
A
A
A
There were large seasonal variations in soil solution during the 2003-2004
sampling seasons. The 2003 growing season was characterized as normal to dry
with a major storm event in the middle of August, resulting in a tenfold increase in
stream flow for a couple of hours. This resulted in a large response in the stream
water chemistry, which dominated the seasonal pattern and could be traced by
rapid changes in concentration for most of the different parameters measured
(paper IV). The 2004 season was dry, lacking heavy rain events and thus any large
fluctuations of the stream water chemistry (papers II and IV).
4.3.1 pH and DOC
The pH increased with depth in both arenosol and podzol in soil solution
recovered with both centrifugation- and lysimeter sampling. The pH was higher
and DOC lower in the three lowest soil horizons of the podzol compared to
corresponding horizons in the arenosol for both 2003 and 2004, demonstrating the
impact of DOC on pH (papers II and III).
DOC generally decreased with soil depth in both recharge- and discharge areas,
but the decrease was more pronounced in the recharge area, likely due to the
constant vertical transport in this soil, inducing this pattern through filtering,
adsorption or degradation across the vertical soil profile.
In the lysimeter samplings pH and DOC were usually low during the first
sampling occasion in early spring, likely due to acidic water from the snow melt
penetrating the soil in combination with frozen conditions and thus lack of contact
with the soil organic matter. During the rest of the seasons there was a tendency
towards increasing DOC, due to increased degradation of organic matter and
microbial activity, and also decreasing pH, as a consequence of increased DOC and
nutrient uptake by the flora (paper II).
During the driest periods in the middle of summer 2003 and in the end of
summer 2004, pH in the lower horizons peaked as there was no water percolating
downwards from the upper and more acidic horizons (paper II).
The pH and DOC in Fanbergsbäcken was usually in the same range as the
deeper soil horizons in the discharge area, indicating these as a main contributors
to stream water chemistry, except for in early spring and the storm event in 2003
where increased water content in the soil and presumably overland flow resulted
in the stream water mirroring the more superficial soil layers (papers II and IV).
27
Fanbergsbäcken had a significantly lower pH and higher DOC content than
Gråbergsbäcken. The pH and DOC were negatively correlated in both streams
with a major decline in pH and a corresponding increase in DOC occurring during
the storm event in 2003. In 2004 as well there was a significant correlation between
these parameters in Fanbergsbäcken but not in Gråbergsbäcken (paper IV).
Earlier studies of stream water chemistry during spring flood and autumn
storms in northern Sweden have shown that DOC is the most important driving
mechanism of the pH decline in forested catchments (Laudon & Bishop 2002). The
organic acidity is thus of major importance for the pH of the stream water. At high
flow, superficial flow paths that activate new sources and an increased washout of
DOC have been proposed as the reason for the large increase in DOC (Bishop &
Pettersson 1996). The same patterns were observed in both streams in this
investigation, and DOC was significantly correlated between the streams in both
2003 and 2004 and pH in 2003, which indicates that the same response was
triggered by the precipitation, and that the process was independent of the
different concentrations of DOC (paper IV). The flow induced by the precipitation
governs the concentration of DOC as also found by Haaland et al. (2010) and Riise
et al. (1994).
4.3.2 DOC fractionation
The 2003 stream water and the soil solution and stream water from the
centrifugation sampling in 2004 were subjected to fractionation of DOC (papers III
and IV). There were large differences in size distribution between the rechargeand the discharge area in the soil. The recharge area had a higher relative
proportion of LMM DOC (paper III). This is in accordance with the podzolisation
theory, where LMM compounds play a crucial role. The relative amount of HMM
DOC also decreased with depth in accordance with the findings by Riise et al.
(2000), who suggested that HMM DOC were filtered or adsorbed when passing
through the podzolic soil. In the discharge area the fraction of HMM DOC was
higher, likely due to different flow conditions (less vertical filtering) and
waterlogged conditions for parts of the year, giving rise to anaerobic environments
slowing down the degradation of the HMM substances into smaller sizes (Figure
6).
The DOC concentration was much lower in stream water than in the soil
solution. The relative amount of HMM DOC in stream water, however, sampled at
the same time as the centrifugation sampling of soil solution was made, was in the
same range as in the soil solution of the shallower soil layers of the arenosol
28
indicating a close relationship between the discharge area and the stream water.
During 2003 the fractionation of DOC from both streams showed high amounts in
the HMM fraction, approximately 70 % in Fanbergsbäcken, while it was somewhat
lower, around 60 %, in Gråbergsbäcken. The concentration of both DOC and HMM
DOC was significantly correlated between the streams (paper IV).
Figure 6. Size distribution for DOC in soil solution from centrifugation sampling 2004 in
the recharge (left) and discharge area (right).
4.3.3 Carboxylic content
The carboxylic concentration was estimated by titration of soil solution from the
centrifuged samples and of stream water. There was a high content of carboxylic
groups in the organic horizons that were not associated with cations recovered in
the HMM fraction, which indicates the presence of free carboxyl groups or
carboxyl groups belonging to the LMM fraction (Figure 7). In the mineral horizons
of the arenosol, HMM organics increased with depth. Here, there were no
unassociated carboxylic groups and thus no LMM complexes. Instead an anion
deficit (AD) was found indicating the presence of HMM inorganic complexes
(Figure 7).
29
Figure 7. The total carboxylic content (striped bar) and the cations recovered in the high
molecular fraction of DOC (stacked bars) in soil solution from centrifugation sampling
2004 in the recharge (left) and discharge area (right).
The carboxylic concentration was correlated between the streams for both
sampling seasons and also to DOC in both streams, but showed a higher
concentration (µeq/l) in Fanbergsbäcken in accordance with the higher content of
DOC in this stream. The difference in site density for the streams was large (Table
4). Fanbergsbäcken was well within the range of the universal model proposed by
(Oliver et al. 1983) and of that reported for Swedish sites by Bergelin et al. (2000)
and Hruska et al. (2003).
In Gråbergsbäcken the temporal variation of site density was greater with
higher averages. The discrepancy present between site densities of the streams
might be attributable to the large differences in DOC concentration and
fractionation. In Gråbergsbäcken the relative fraction of LMM DOC was larger,
indicating a greater presence of fulvic compounds with higher charge density
(COOH/C-ratio; Stevenson 1993). It should be noted however that, compared to
Fanbergsbäcken, the low DOC concentration and higher concentrations of the
strong acid anions F and NO3 in this stream would yield a higher uncertainty for
the calculated site density values due to error propagation (Hruska et al. 2001). The
highest site density notation was for Gråbergsbäcken in 2004, which was a very
dry year with the lowest DOC values detected.
30
Table 4. Site density for stream water (µeq/mg C) in the outlet of the Fan- and Grå
catchment, the third sampling point in Fanbergsbäcken (Fan3) and Gråbergsbäcken (Grå3).
Fan3
Grå3
2003
2004
2005
2006
average
10.02
9.40
9.49
8.72
stddev
1.47
3.03
1.81
1.99
n
36
28
23
27
average
14.13
20.83
15.73
14.07
stddev
6.84
13.95
9.39
6.93
n
36
20
20
30
4.3.4 Low molecular mass organic acids (LMMOAs)
LMMOAs are small acids that are ubiquitous in soil solution and have high
complex binding abilities to cations and metals. They play an important role in soil
processes such as weathering of minerals in the podzolation process (van Hees &
Lundström 2000) and are recovered in the LMM fraction of DOC (<1 kDa). Using
the method developed in paper I, LMMOAs were analyzed in both soil solution
from centrifugation and stream water.
There was no significant difference in the amount of LMMOAs recovered in the
podzol and the arenosol, which was expected due to the difference in DOC
concentration. The LMMOAs comprised <3.5 % of the DOC in the podzol, and <1.5
% of the DOC in the arenosol.
The most common LMMOAs recovered in the soil solution were, in declining
order, citric-, malic- and shikimic acid (Figure 8). All acids, except lactic acid, were
found in the highest concentration in the O-horizons and diminished with depth in
the soil profile, similar to the DOC concentration (Figure 6).
31
Figure 8. The vertical distribution, average values with standard error (n=6), of the three
most common LMMOAs in soil solution from the centrifugation sampling in 2004.
There have been few investigations of LMMOAs in stream water, possibly
because of the low concentrations present in this medium. The method developed
in paper I proved to be sensitive enough for this purpose, however. LMMOAs
were measured in Fanbergsbäcken during 2003 and 2004 and Gråbergsbäcken
during 2003-2007. The total amount of LMMOAs did not differ significantly
between the streams in 2003, but in 2004 it was higher in Fanbergsbäcken.
The most common acids in the stream water were oxalic- and lactic acid, of
which peak concentrations of oxalic acid coincided with those of DOC. Oxalate in
the stream water in 2004 was in the same concentration range as found in the Olayers of the discharge area/arenosol in the centrifugation sampling the same year.
In Gråbergsbäcken the seasonal variation for DOC and oxalate were correlated
(p<0.001) for the entire investigated period (Figure 9), while no such correlation
was seen in Fanbergsbäcken during 2003-2004 (although a positive correlation
could be seen for oxalic acid in the first two sampling points in Fanbergsbäcken
2003). There was no relation between the concentrations of DOC and lactate in
either of the streams, which seems consistent with its behavior in soil. Some of the
most common acids in the soil solution, shikimate, citrate and malate, were hardly
ever found, and only then in very low concentrations in stream water, and
therefore did not seem to make the transition from soil to stream water in the same
way as oxalic acid did.
One of the few investigations of LMMOAs in streaming water by Jonsson et al.
(2007) reported much higher concentrations of citrate than found in this
investigation, and also a negative correlation of LMMOAs to DOC in the inlet and
32
outlet of a subarctic lake in northern Sweden. This indicates a very different
LMMOA constitution compared to the present investigation, and highlights the
influences of the environment on these substances.
Figure 9. Seasonal variations for DOC (n=147) and oxalate (n=124) in Gråbergsbäcken
during 2003-2007.
4.3.5 Base cations and their association to DOC
The association of cations and metals to DOC was investigated in the stream
water in 2003 and in the soil solution sampled by centrifugation in 2004.
The monovalent base cations, Na and K, were not significantly associated to
HMM DOC in either soil solution or stream water. Na and K were most likely
present as free ions and experienced only weak or no association to DOC. K
showed large variations in the O- and E-horizon in the soil solution from lysimeter
sampling due to high recirculation of this substance in biota (paper II) while Na
that was more abundant, being less susceptible for plant uptake, was found in
higher concentration in the mineral soil. Both substances showed only minor
variations in the stream water over the season (papers II and IV).
33
Ca and Mg were found in the highest amounts in the topsoil solution and then
diminished with depth in the soil profile in both the recharge- and the discharge
area. During the summer months both Ca and Mg tended to increase in the soil
solution, probably due to weathering and degradation of biota, and then a decrease
was seen in the autumn due to wash out with the increase in precipitation and
decreasing microbial activity, leading to a lower weathering rate (papers II and III).
Fifty percent of the Ca was found to be associated to HMM DOC in solution
throughout the soil profile both in podzol and arenosol, except for the organic
horizon in arenosol where the fraction associated to HMM DOC was 60-80 %
(paper III). The distribution of Mg in soil solution was similar to Ca, except for a
slightly higher HMM fraction of Mg in the mineral soil in the discharge area.
There was a higher concentration of Ca in Gråbergsbäcken than in
Fanbergsbäcken during both seasons, while the opposite was observed for Mg. The
higher concentration of Ca in Gråbergsbäcken might be attributable to the greater
influence of groundwater due to the deeper stream channel in this stream. Both Ca
and Mg were positively correlated between the two streams during 2003 and 2004,
and both were positively correlated with DOC and its HMM fraction in both
streams during 2003, which indicates that these elements have a considerable
association with organic matter in stream water. Surprisingly high amounts of both
Ca and Mg were associated to HMM DOC in both streams during 2003 (Table 5),
and peak values of around 80 % of Ca and Mg associated to HMM DOC were
reached in Fanbergsbäcken during the high flow event in August, with slightly
lower values for Gråbergsbäcken (paper IV).
Such high association of Ca to HMM DOC has previously been reported by
Benedetti et al. (2003) from river water in Rio Negro, Brazil. A similar range was
also found in the soil solution sampled by centrifugation in the Fanbergsbäcken
discharge area in 2004, as mentioned above (papers III and IV). This confirms a
superficial flow in the soil during high flow/high precipitation events where the
stream water adopts the chemistry of the soil it was last in contact with (similar to
findings by Bishop et al. (2004)). Despite this, Dahlqvist et al. (2004) reported a
considerably smaller association of Ca to DOC in a study of river water from
Kalixälven in northern Sweden, and noted a risk of overestimation of the HMM Ca
with use of conventional ultra filtration methods, as used in this study. On the
other hand, DOC quantity and quality differs substantially between headwaters
and large rivers and also between catchments. This subject therefore requires
further investigation.
34
Table 5. The median value (1st - 3rd quartile) of the fraction of the different substances
recovered in the HMM-fraction (Mw>1 kDa) in the third sampling point (the outlet of the
catchments) of Fanbergsbäcken and Gråbergsbäcken during 2003. Sign tests (Miller &
Miller 2000) were performed on seasonal series to evaluate differences between the streams.
DOC (n=29,31)
Ca (n=17, 17)
Mg (n=17, 17)
Al (n=17, 17)
Fe (n=17, 17)
Fan3
68% (55-81%)
54% (47-67%)
48% (36-60%)
73% (61-86%)
79% (73-87%)
Grå3
58% (50-69%)
35% (23-45%)
24% (19-37%)
67% (50-82%)
40% (0-71%)
P (sign test)
0.043
0.013
0.057
1
0.023
4.3.6 Al and Fe and their association to DOC
In the recharge area with a clear podzol profile, the concentrations of Al and Fe
in solution were decreasing with soil depth in the mineral soil due to
immobilization/precipitation. Fe was found almost entirely in the LMM fraction in
the B-horizon while in the other horizons the association to HMM DOC was
around 70 % (Figure 10). Fifty percent of the Al was recovered as HMM in the Ohorizons, while the association was slightly higher (~75 %) in the mineral horizons
(Figure 10;paper III).
Figure 10. Size distribution for Fe and Al in soil solution in the recharge area from the
centrifugation sampling 2004.
An increasing trend with depth in the arenosol soil profile was visible for Al
and Fe, and both substances were found in highest concentrations in the deep
mineral soil solution. Compared to podzol, a higher proportion of Al and Fe were
bound to HMM organics, highest in the shallow mineral soil (0-5 cm) where the
association was 95 % for Fe and 90 % for Al.
35
The seasonal variations of Al and Fe concentration in soil solution were
somewhat less clear than those for Ca and Mg, but generally followed the same
pattern with increases during the summer months and a decrease in the autumn.
The highest concentrations for both substances, however, were usually recovered
in the E- and 0-5 horizons in the lysimeter investigation, clearly differing from the
centrifugation investigation of the arenosol (Figure 11). Previously, differences in
the recovery of different elements with different sampling techniques i.e.
centrifugation and tension lysimeters among others, has been described by Geibe
et al. (2006).
Figure 11. Size distribution for Fe and Al in soil solution in the discharge area from
centrifugation sampling 2004.
Both total Fe and total Al were found in significantly higher concentrations in
Fanbergsbäcken than in Gråbergsbäcken, and the concentration of Al was
substantially higher than Fe in both streams (paper IV). Total Fe and Al were
significantly correlated with DOC in both streams during 2003 and 2004, except for
Fe, which was not correlated to DOC in Gråbergsbäcken during 2004 likely due to
the low concentrations and lack of high flow events. The correlations emphasize
the importance of DOC for the transport of cations and metals in stream water.
Fanbergsbäcken had a significantly higher amount of Fe associated with the
HMM fraction (Table 5). The lower association of Fe to HMM DOC in
Gråbergsbäcken was possibly due to higher fraction LMM DOC and the low total
concentration of Fe. Björkvald et al. (2008) and Kortelainen et al. (2006) have shown
that the hydrogeochemistry of Fe is highly dependent on the catchments coverage
of wetlands, and in this investigation the greater abundance of wetlands lining the
36
stream channel in Fanbergsbäcken might explain the significantly higher
concentrations of Fe in this stream.
During the 2003 season, a majority of the Al in the streams was found
associated to the HMM fraction of DOC (Table 5), in accordance with previous
studies of the behaviour of Al in soil solution (Riise et al. 2000; van Hees et al. 2001;
paper III). There was no significant difference in the association of Al to the HMM
DOC between the streams (Table 5). The HMM fraction of Al was strongly
correlated to both DOC and total Al in both streams. During 2003, the
concentrations of Fe and Al were positively correlated between the streams, but in
2004 no such correlation was noted. This is presumably a direct effect of the low
concentrations of these substances in combination with the absence of high flow
events during the 2004 season.
5. IN-STREAM VARIATIONS
The in-stream variation was more pronounced in Fanbergsbäcken where many
of the measured parameters showed significant differences between all three
sampling points, while in Gråbergsbäcken differences were mostly found between
the last two sampling points. In-stream differences were most noticeable for pH,
where Fanbergsbäcken showed a significant decrease between all three sampling
points downstream in both 2003 and 2004, in accordance with a significant increase
of DOC. In Gråbergsbäcken a significant increase in pH occurred between all three
sampling points in both years, without the corresponding decreases in DOC.
Fanbergsbäcken showed higher concentration values downstream for Al and Fe
as these substances were highly associated to DOC in this stream. In
Gråbergsbäcken, Fe increased between all three sampling points downstream,
while DOC, Ca and Al increased between the second and the third sampling point
(Table 6). The increase of Fe and Al and the decrease of pH downstream in
Fanbergsbäcken is clearly a consequence of the increasing DOC concentration; this
is also visible in Gråbergsbäcken between the last two sampling locations for Fe, Al
and possibly Ca (paper IV).
37
Table 6. Differences (paired t-test, P<0.05) for in-stream variations between sample points
in Fanbergsbäcken and in Gråbergsbäcken during the 2003 sampling season. should be
interpreted as an increase and as a decrease between the sampling points.
Fan1 Fan2
Fan2 Fan3
Grå1 Grå2
Grå2 Grå3
pH
DOC
tot-LMMOAs
≡
Ca
Al
Fe
Cl
≡
≡
≡
≡
≡
≡
≡
F
≡
≡
≡
≡
6. WOOD ASH APPLICATION (WAA)
6.1 The effect of WAA on stream water- and soil solution chemistry in the
Fanbergs catchment
Spatially uniform application of wood ash in forests is nearly impossible, due to
the variable cover and difficulty of access. Irregularities in the ash distribution
were avoided to a certain degree at the sampling site, as the ash was spread by
hand. However, a perfect evenness of the ash layer was not certain and the
lysimeter sampling, representing a small volume of soil solution close to the
installation, may therefore reflect this. To compensate, the samples from three
lysimeters were pooled at each pit and soil solution concentrations were averaged
for three pits representing, at the most, nine lysimeters for each horizon and each
“treatment” (ash podzol, ash arenosol, control podzol and control arensosol).
Monitoring the water chemistry in the last sampling point in Fanbergsbäcken is
representative of the discharge processes from the whole catchment, 50 ha, where
150 ton of wood ash had been applied.
6.1.1 Respiration
Soil respiration measurements were not different between the ashed area and
the control area, and varied in the range of ~1-12 µmol CO2/m2s over the sampling
season (Figure 12). In 2005, the first year after WAA, the respiration seemed to
increase, but this increase was present in both the ashed area and the control area.
No change in respiration could therefore be related to the WAA in this
investigation.
38
Figure 12. Respiration measurements (µmol/m2s) 2003-2006 in the recharge area (a) and
the discharge area (b) in the Fanbergs catchment.
6.1.2 pH and DOC
No difference in pH resulting from WAA was found in soil solution at any
depth in either the podzol or the arenosol in this investigation. No difference in
DOC was noted either, even though a tendency towards an increase could be seen
in 2005 and 2006 for the O- and 0-5 horizon in the arenosol. High seasonal
variations, foremost in the superficial soil layers, made it difficult to distinguish
any effects caused by the WAA from the natural variations (paper VI). The pH of
the treated area’s stream water after WAA remained unchanged.
A significant increase in DOC concentration in Fanbergsbäcken compared to
Gråbergsbäcken was observed, however. The lack of corresponding decrease in pH
may indicate a pH-related change since this shows a deviation from previously
established strong negative relationships between pH and DOC in these streams
during the 2003-2004 pre-treatment period (paper IV).
The WAA did not affect the total LMMOAs concentration in Fanbergsbäcken
and there was no difference during the investigated period in Gråbergsbäcken
either. During the four sampling seasons, oxalic acid was the most dominant acid
in Fanbergsbäcken whereas it alternated between oxalic- and lactic acid for
Gråbergsbäcken. Malonate was the only of the 17 analysed LMMOAs that seems to
be affected by WAA. There was an increase in Fanbergsbäcken when looking at the
average differences (paper V). Data on individual LMMOAs in stream water are
scarce in the literature, but malonate has previously been found to increase in soil
solution in superficial soil layers at a forested site in southern Sweden due to
treatment with 8.75 tonnes/ha dolomite (Holmström et al. 2003), indicating that
39
changes in the production of this organic acid can be induced by catchment scale
treatments with ash or lime.
No increase could be seen in the total carboxylic concentration of the stream
water in the present study, despite an increase in DOC, which would imply a shift
towards more hydrophobic DOC as previously observed by Holmström et al.
(2003). No decrease in site density was found to confirm this, however. This means
that the small increase of carboxylic groups compensated the increase in DOC and
WAA did not induce any significant changes in the quality of the DOC.
Increased DOC concentration in soil caused by application of lime and wood
ash has been observed in several studies and attributed to a fertilization effect
occurring from the treatment that increases the biological activity and therefore the
turnover time for different carbon compounds in the soil (Nilsson et al. 2001;
Perkiömäki & Fritze 2002).
In a study of both soil solution and stream water chemistry over a five-year
period following the application of 2.2 ton/ha of granulated fly ash to a small
catchment area, Fransman & Nihlgård (1995) found an increasing pH tendency in
runoff, while no significant change was seen in soil solution at 30 cm depth. A
slight increase of the pH in a brook running through mineral soil was also noted by
Tulonen et al. (2002) one year after application of 6.4 ton/ha of wood ash to a small
subcatchment.
Geibe et al. (2003) observed higher pH in the O2 horizon, but lower pH deeper
in the soil horizon in comparison to a control area four years after application of
4.28 ton/ha of wood ash, possibly due to ion exchange of protons with applied
cations. Higher DOC concentration was reported in the topsoil of the sites
subjected to WAA, compared to the control (Geibe et al. 2003; Holmström et al.
2003).
When investigating WAA to a pine stand in northern Germany, Rumpf et al.
(2001) noted an initial decrease in the pH of superficial soil layers in a sandy
podzol after application of 2.4 ton/ha wood ash. Twenty-four months after WAA,
pH in the treated plots was still lower than in the control. This indicates the same
ion exchange effect as noted by Geibe et al. (2006) through WAA.
Ring et al. (2006) observed an increased pH in soil solution at 50 cm depth four
years after application of 9 ton/ha of crushed wood ash, and elevated TOC
40
concentration in soil solution one year after application of 3, 6 and 9 ton/ha. Other
studies have also noted increased pH-values in the soil of superficial soil layers
after application of different doses of wood ash to forest soil (Eriksson 1998a;
Jacobson et al. 2004; Saarsalmi et al. 2001a).
Increased pH of stream water is a desirable effect since it may reduce
mobilization of inorganic Al. In this investigation we found no increase of pH and
no reduction of Alqr or total concentration of Al after WAA in either soil solution or
stream water. The species in the ash foremost responsible for pH increases are OH
and HCO3, however large amounts of base cations may induce an increased ion
exchange, releasing exchangeable protons and possibly inorganic Al. This would
lead to a more acidic soil solution as suggested by Lundström et al. (2003b), and
the effect may therefore be a potential suppression of the increase in pH (paper VI).
6.1.3 Ca and Mg
Lysimeter soil solution data for cations and metals were fewer in quantity than
for pH and DOC, especially in the superficial soil layers, due to periodically dry
conditions in combination with the requirement of a larger sample volume. This, in
combination with large seasonal variations, made it hard to achieve statistical
significance and results are reported as “trends” or “indications” in some cases.
Large amount of base cations were added to the soil with the wood ash and
foremost was Ca, which compared to the area closest to the stream, was added in
amounts more than 30 times (g/kg DS) higher than the amount present in the most
superficial mineral soil layers (Table 2). Effects of WAA on Ca or Mg
concentrations were hard to distinguish by visually examining the data pattern,
but t-test analysis revealed differences not visible before the WAA appeared for Ca
in the mineral soil of the arenosol in 2005 (0-5; p=0.03, 10-15; p=0.003, 45-50; p=0.02;
paper VI). This indicates an increase in the ashed plot compared to the control plot
after WAA. This enhancement in Ca-content was still visible in the 0-5 horizon in
2006 (p=0.04). There were no detectable changes in Mg following WAA.
Geibe et al. (2003) reported significant increases of both Ca and Mg in soil
solution achieved by centrifugation for the whole soil profile four years after
WAA, and a similar result was observed for areas treated with dolomitic lime. The
same investigation found that Ca was retained to a greater extent in the shallower
soil layers, while Mg was more mobile and recovered to a greater extent in deeper
soil layers. Higher mobility for Mg than for Ca have also been reported in limed
plots by van Hees et al. (2003). Ring et al. (2006) found only elevated Ca in soil
41
solution from lysimeters however, from the highest wood ash dose (9 ton/ha) four
years after application.
The stream water investigation yielded much higher resolution than soil
sampling, which made it easier to distinguish small changes in concentration.
There were significant increases in both Ca and Mg after WAA (paper V; Figure
13), which is perhaps not that surprising considering that large amounts of these
substances had been added to the catchment (Table 2). Both substances in the preinvestigation were associated to DOC in a similar fashion (papers III and IV), and
the increased concentration of DOC in Fanbergsbäcken seen after WAA might
partly explain the increase of Ca and Mg in the stream. The addition of Ca to the
catchment was far greater than that of Mg however - 30 ton Ca in total compared
to 2 ton Mg - while the percent increase in stream water after WAA was similar for
both substances (paper V). This indicates a somewhat faster release of Mg from the
ash.
Fransman & Nihlgård (1995) observed an increase in Ca, although not
statistically different from the reference, in a stream draining a treated catchment;
no change was found in soil water from 30 cm depth though. Mg in stream water
remained unchanged as a result of WAA, but an increase was noted in stream
water in a limed catchment, the latter confirmed by Hindar et al. (2003). Tulonen et
al. (2002) observed no change in Ca in two brooks draining a small wood-ashed
area. Steenari et al. (1998) noted total Ca losses of up to 40 % in both a simulated
weathering study, and when extracting weathered ash particles from the forest
floor two years after WAA, while a much slower leaching rate was reported for
Mg.
Figure 13. Mg in Fanbergsbäcken and Gråbergsbäcken 2003-2006 (a) and cusum chart of
differences (b). The vertical line denotes WAA to the Fanbergs catchment.
42
6.1.4 K and Na
Large variations in K concentration were visible in the soil solution of all
sampled plots in 2005, although these variations were slightly larger in the ashed
area (paper VI). The concentration of K increased significantly in the O-horizon of
the podzol (p=0.009), and there were indications of elevated concentrations
compared to control even in 2006. K is highly mobile and is known to increase in
soil initially after WAA (Kahl et al. 1996). Ring et al. (2006) found significant
increases in K in soil solution from 3, 6 and 9 ton/ha of crushed wood ash, and the
effect persisted for 4, 7 and 9 years respectively after treatment. Geibe et al. (2003)
found an increase in K at a depth of 25-30 cm when investigating soil sampled by
centrifugation. Several studies have also reported increases in exchangeable K after
WWA (Jacobson et al. 2004; Saarsalmi et al. 2001b) but the increase was usually
located in a deeper horizon or disappeared faster than the increase in Ca due to the
higher mobility of K (Kahl et al. 1996).
In stream water K concentration showed the most pronounced increase after
WAA. This is clearly distinguishable in the cusum chart in the spring 2005 and
2006 and at a high flow event in the middle of the 2006-season (paper V). Eriksson
(1998b) demonstrated in a laboratory study that K leached very rapidly from the
mor layer, implying weak interactions with organic soils. This may explain the
large increases that can be seen at high flow, as K is washed out from superficial
soil layers. Jacobson et al. (2004) verified this behaviour for K in soil 5 years after
application of wood ash, where increased concentrations of exchangeable K were
found much deeper in the soil profile than Ca and Mg for instance, where the latter
substances increased in the mor layer. Nieminen et al. (2005) observed significant
decreases in K in investigations of ash particles 3 and 5 years after ash application
to the topsoil layer. K is thus released very quickly from the ash particles and, as
this study demonstrates, also reaches the runoff shortly after ash application to
forest soil. Tulonen et al. (2002) and Fransman & Nihlgård (1995) observed similar
increases in K in stream water, which seems to be the most mobile of the cations
and therefore shows the strongest leakage pattern.
No significant changes in Na concentration were detected in either soil solution
or stream water, which was unexpected as Na is known to increase soil and soil
solution after WAA (Kahl et al. 1996; Ring et al. 2006). In this investigation loose
wood ash was spread in late September, possibly leading to a low penetration of
the soluble substances of the soil before the frost, which would contribute to a large
washout in early spring from the soil surface or superficial soil layers, similar to
that seen for K. Na is more abundant however, and is less limiting nutrient in
43
forest soils and therefore would probably leach to deeper soil layers at a faster rate
than K. This in combination with a lower amount off Na added with the ash would
make any possible effect too low to distinguish.
6.1.5 Transition elements and other metals
The only effect of WAA on the other metals analyzed in the soil solution was an
increase of Fe compared to the control in the E horizon of the podzol in 2006 (paper
VI). Fransman and Nihlgård (1995) observed a decrease in Fe in soil solution from
30 cm depth after application of 2.2 ton/ha of wood ash. They attributed this
decrease to the increase in pH, making Fe less soluble. Reductions of Fe in water
from brooks draining a wood ash treated (6.4 ton/ha) area have also been reported
by Tulonen et al. (2002). There were no increases of any metals in stream water in
that investigation.
6.1.6 Anions
Anions are present in high amounts as counter ions in the ash, and Cl and SO4
have most prominently shown rapid initial leaching from wood ash in experiments
(Steenari et al. 1998). No ash-related effect was seen for Cl in any of the
investigated soils although the seasonal variations were very high and may have
been masking potential effects (paper VI). Cl was however the only anion observed
to substantially increase after WAA in the stream (Figure 14; paper V)), similar to
findings reported for stream water by Tulonen et al. (2002).
Figure 14. Cl concentration in Fanbergsbäcken and Gråbergsbäcken 2003-2006 (a) and
cusum chart of differences (b). The vertical line denotes the WAA to the Fanbergs
catchment.
A higher concentration of SO4 in the three mineral horizons of the ashed
arenosol was detected during 2005 (paper VI). Of these horizons, the 10-15 horizon
showed higher concentrations already in 2003 and 2004, but the increase in 0-5 and
44
45-50 horizons was newer (p=0.009, p=0.012). In 2006, the only notable increase was
in the O-horizon. Somewhat unexpectedly, SO4 remained unchanged in stream
water. Previous observations have shown increases of SO4 in soil solution after
WAA (Ring et al. 2006) and it was also detected in high concentration in initial
leaches in a laboratory leaching experiment of self-hardened crush ash by Steenari
et al. (1999). Ludwig et al. (2002) noted large increases in SO4 concentration in soil
solution at in a pine stand treated with 4.8 ton/ha of stabilized wood ash. Two
years after the application the concentration of SO4 was still higher at 0, 10 and 100
cm compared to the control plot.
Higher F concentrations were found in the O, 0-5 and 10-15 in the wood ashtreated arenosol compared to control during 2005, probably due to a slight
decrease of this substance in the control area. A tendency towards an increase in
the ashed arenosol was seen for 2006 however, but due to lack of samples and high
standard deviations this increase was not statistically confirmed. F was the only
anion that decreased in stream water after WAA (paper V).
There was an increase in AD in stream water as an effect of WAA (paper V).
The AD consists of bicarbonate, hydroxyl groups and carboxyl groups, both free
and associated to cations. These substances are all potential proton acceptors,
depending on the pKa of the substance and the complex binding characteristic, and
may thus contribute to the buffer capacity of the stream water. There was no
change in pH after WAA, while the carboxylic concentration only tended towards
increase. This would imply that the conditions for cation association to DOC had
not changed substantially, although Ca and Mg had increased slightly. A
significant amount of the Ca, Mg, Al and Fe in the stream water had previously
been shown associated to organic matter (paper IV). That leaves mainly
bicarbonate to account for the possible increase in buffer capacity, and the effect on
the buffer capacity is likely smaller than the observed change in AD concentration.
6.1.7 Si
The concentrations of Si varied in soil solution, with occasional peak values,
most likely due to dry conditions, making the data hard to interpret. The lowest
concentrations were found in the O-horizon while higher concentrations of Si were
found in the mineral horizons. Si concentration was significantly higher in the
ashed podzol plot compared to the control plot in 2006.
45
Si increased considerably in the stream water in the treated area (Figure 15),
which is hard to reconcile with the results of Steenari et al. (1999), who identified Si
in stabilised wood ash in the form of highly insoluble silicates and quartz. They
also found very low concentrations of Si in leachates. To the author’s knowledge, a
major increase of Si in runoff after WAA has not previously been reported.
Figure 15. Si in Fanbergsbäcken and Gråbergsbäcken 2003-2006 (a) and cusum chart of
differences (b). The vertical line denotes the WAA to the Fanbergs catchment.
6.2 In-stream variations in Fanbergsbäcken
There were no visible effects of WAA through comparison of the in-stream
variation, but of note is that K, which had a decreasing trend downstream during
2003-2005, showed an increase downstream in 2006. Other noteworthy changes
include the decreases in Na and Ca between all sampling points in 2004 and 2005,
which later became increases between the second and third sampling point in 2006
for Na, and a tendency towards an increase for Ca. The in-stream effects seen for K,
Ca and Na in 2006 might be secondary effects caused by ash constituents reaching
the stream through different flow paths, rather than the effects discussed hitherto.
The ash effect on water subjected to slower hydrological pathways with longer
residence times in the soil have begun to contribute to the stream chemistry at the
end of this investigation, and the impact of the wood ash may therefore have
entered a new phase after this initial time span. The effects are mild and may easily
be missed in an investigation with lower resolution.
7. CONCLUSIONS AND FURTHER PERSPECTIVES
Most of the studied elements in soil solution, especially DOC, Ca, Al, SO4, and
Si increased in concentrations during the growing season due to increased
46
weathering and increased degradation of biota and litter. These seasonal patterns,
however, were not recognized in the concentrations in stream water. The stream
water more likely reflected the soil horizons from which the water was received
during different flows and was thus more dependent on the soil solution
concentrations at different soil depth.
In the recharge area, Al and Fe were precipitated in the ongoing podzolisation
process as indicated by the low concentrations recovered in the deep mineral soil,
and Al and Fe were to a greater extent associated to LMM DOC than in the
discharge area and stream water, where they were mainly associated to HMM
DOC. Hence, there was a shift in the DOC composition throughout the catchment.
In the discharge area the soil forming process was strongly affected by the HMM
organic matter and the highest concentrations of Al and Fe were found in the
deeper mineral soil.
It has been shown that the quantity and quality of the DOC differed greatly
between the streams, with higher DOC, total caboxylic concentration and a higher
concentration and percentage of HMM-DOC in Fanbergsbäcken, resulting in a
higher association of cations to HMM-DOC in this stream.
High fractions of Ca and Mg were associated to HMM DOC in stream water of
two first order streams draining bordering catchments. This, however, has not
been reported in other studies and needs further investigations. An investigation of
the associations of metals to HMM DOC using SEC-ICP-MS is planned for 2011.
Even though there were large differences between the concentrations of DOC
and associated cations in the streams, the seasonal patterns showed significant
correlations indicating that precipitation induced changes in flow that triggered
similar responses in the stream water chemistry of both catchments. The flow
pattern driven by precipitation therefore seems to be the driver of the stream water
chemistry. This made it possible to use Gråbergsbäcken as an untreated reference
when investigating the effect of a catchment scale wood ash application on the
water chemistry of Fanbergsbäcken.
A selective method identifying and quantifying 17 different LMMOAs down to
nM-concentrations has been developed and thoroughly tested. The method is fast
and efficient and has so far been tested on a wide range of media, including stream
water, soil solution sampled by lysimeters and by centrifugation and growth
medium for ectomycorrhizal fungi with good performance. To further improve the
47
method, the addition of a new mass track for isocitrate is planned, to make it easier
to separate the structural isomers citrate and isocitrate.
There was large divergence in the constitution of LMMOAs in soil solution and
stream water, with some of the acids recovered in high concentrations in soil
entirely absent in the stream water samples, while others were recovered in the
same order of magnitude in both mediums. LMMOAs do not seem to make up a
constant fraction of DOC in either soil solution or stream water, and further studies
are needed to investigate processes influencing the behavior of LMMOAs in the
transfer between and interfaces of these mediums.
The Swedish Forest Agency recommends WAA to counteract acidification in
soil and runoff. The effect of wood ash on soil solution was small. Temporary
increases of K, Si and Fe concentrations in soil solution compared to the control
were detected in the recharge area, as well as for Ca and SO4 in the discharge area
when comparing ashed and control areas.
Investigation of the initial effects of WAA to a first order stream revealed no
effect on pH, but a significant increase in base cations balanced by leaching of Cl
and an increase of the anion deficit, interpreted partly as an increase in buffer
capacity. The DOC concentration increased, probably as a response to raised
microbial activity in the soil, even though this was not seen in the respiration
measurements, leading to enhanced leaching. As a tool to counteract acidification
of surface waters, WAA seems to have limited initial effects, raising the buffer
capacity slightly but not affecting the pH. Small but statistically significant effects
could be reported, by means of high sampling frequency and enhanced by
cumulative sum chart, which, in combination with Students t-test and RIA, proved
to be useful in investigating time trends in stream water.
In these investigations, chemical analysis of stream water seems like a more
representative method of evaluating the effect of WAA to a catchment than
lysimeter data. Stream water represented the whole catchment to which 150 tonnes
of wood ash had been applied, while lysimeters represented a very small soil
compartment immediately surrounding the lysimeter. Unevenly distributed wood
ash would therefore have far greater impact on the soil solution than on the stream
water chemistry in this case. A long series of continuous data is also much easier to
achieve when sampling stream water than soil solution, since the former is less
sensitive to drought.
48
So, what is the most important that I have learned? I think it’s best summarized
by Susan Sto Helit while reading a bedtime story
“…and then Jack chopped down what was the world's last beanstalk, adding
murder and ecological terrorism to the theft, enticement and trespass charges
already mentioned…” (from “Hogfather” by Terry Pratchett).
That is, some tales are told so many times that they almost turn into facts. But
that does not mean they’re the truth.
49
8. ACKNOWLEDGEMENTS
Thanks to...
Ulla Lundström for giving me the opportunity to do this work, for her never
ending enthusiasm and for introducing me into new projects.
Dan Bylund for sharing his extended knowledge in analytical chemistry and
statistics, for his patience with students that needs more than one explanation and
for his excellent memory.
Jenny Vestin, a fantastic co-worker, a great friend and a machine in the forest and
in the lab. We have made a lot of trips back and forth to Bispgården, shared a lot of
laughs and fika breaks, and I still have your orange.
Mattias Fredriksson for sharing the misery of writing a thesis, thing would have
been a lot harder and more boring without your company.
Andreas Aronsson for sharing his extended knowledge of limnology, and nature at
large (and some football, fly fishing, and roofing).
Kei Nambu for lots of help in the lab, and to Pelle Mellander for great company in
the forest and sharing his knowledge about hydrology.
Madelen Olofsson for bringing some more fun into our group, for great company,
friendship and crazy ideas.
Former and present members of the analytical- and soil chemistry group Tara Ali,
Sofia Essén, Sara Holmström, Rickard Thurdin, Christine Geibe and Tomas
Östlund for valuable cooperation and interesting discussions.
Katarina Brorson, Agnes Hedenström and Martin Eriksson for great help with field
work and in the laboratory.
Torborg Jonsson, Håkan Norberg, Anna Haeggström and Christina Olsson, whose
service-mindedness has been invaluable in different mattes.
Matt Richardson, Ann Almsåker and Nils Ekelund for reading my thesis.
50
All great friends at work; Christina, Ida, Hanna, Anna, Lotta, Per, Johan, Cicci,
Birgitta and many more, who made fika breaks a lot of fun and always showed
support.
Neglected friends; Mari Jönsson, Åsa Kestrup, Åsa Knies, Susanne Büchner and
Lollo and Ceasar Guevara, whose wedding I missed while writing the final version
of this thesis. I hope that I can spend some more time with all of you soon.
Terry Pratchett for making me laugh, when I really needed to.
Love and thanks to Paul and Mary Ann for great support.
Love and thanks to my family, Mor och Jan-Olof, Far och Sylvia, Maria, Tomas,
Jessica och Sofie samt Jakob, Susan, Carmen, Oscar och Ross. Ni är bäst!
And to the pack, for being there
Let’s start spending a lot more time together.
51
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deficiency: a review. Plant Soil 241(1): 75-86
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Anonymous (2008) Rekommendationer vid uttag av avverkningsrester och
askåterföring. In. vol 2/2008. Swedish Forest Agency.
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