1881–1892, www.biogeosciences.net/12/1881/2015/ doi:10.5194/bg-12-1881-2015 © Author(s) 2015. CC Attribution 3.0 License.

1881–1892, www.biogeosciences.net/12/1881/2015/ doi:10.5194/bg-12-1881-2015 © Author(s) 2015. CC Attribution 3.0 License.
Biogeosciences, 12, 1881–1892, 2015
© Author(s) 2015. CC Attribution 3.0 License.
Carbon dioxide transport across the hillslope–riparian–stream
continuum in a boreal headwater catchment
F. I. Leith1,4,5 , K. J. Dinsmore1 , M. B. Wallin2,5 , M. F. Billett3 , K. V. Heal4 , H. Laudon6 , M. G. Öquist6 , and
K. Bishop5,7
1 Centre
for Ecology & Hydrology, Edinburgh, UK
of Ecology and Genetics/Limnology, Uppsala University, Uppsala, Sweden
3 Biological and Environmental Sciences, School of Natural Sciences, University of Stirling, Stirling, UK
4 School of GeoSciences, University of Edinburgh, Edinburgh, UK
5 Department of Earth Sciences, Air Water and Landscape Sciences, Uppsala University, Uppsala, Sweden
6 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), Umeå, Sweden
7 Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden
2 Department
Correspondence to: F. I. Leith ([email protected])
Received: 29 September 2014 – Published in Biogeosciences Discuss.: 7 November 2014
Revised: 19 February 2015 – Accepted: 2 March 2015 – Published: 23 March 2015
Abstract. Headwater streams export CO2 as lateral downstream export and vertical evasion from the stream surface.
CO2 in boreal headwater streams generally originates from
adjacent terrestrial areas, so determining the sources and
rate of CO2 transport along the hillslope–riparian–stream
continuum could improve estimates of CO2 export via the
aquatic pathway, especially by quantifying evasion at higher
temporal resolutions. Continuous measurements of dissolved
CO2 concentrations and water table were made along the
hillslope–riparian–stream continuum in the Västrabäcken
sub-catchment of the Krycklan catchment, Sweden. Daily
water and CO2 export from the hillslope and riparian zone
were estimated over one hydrological year (October 2012–
September 2013) using a flow-concentration model and compared with measured lateral downstream CO2 export.
Total water export over the hydrological year from the hillslope was 230 mm yr−1 compared with 270 mm yr−1 from
the riparian zone. This corresponds well (proportional to the
relative upslope contributing area) to the annual catchment
runoff of 265 mm yr−1 . Total CO2 export from the riparian
zone to the stream was 3.0 g CO2 -C m−2 yr−1 . A hotspot for
riparian CO2 export was observed at 30–50 cm depth (accounting for 71 % of total riparian export). Seasonal variability was high with export peaks during the spring flood
and autumn storm events. Downstream lateral CO2 export
(determined from stream water dissolved CO2 concentra-
tions and discharge) was 1.2 g CO2 -C m−2 yr−1 . Subtracting
downstream lateral export from riparian export (3.0 g CO2 C m−2 yr−1 ) gives 1.8 g CO2 -C m−2 yr−1 which can be attributed to evasion losses (accounting for 60 % of export via
the aquatic pathway). The results highlight the importance
of terrestrial CO2 export, especially from the riparian zone,
for determining catchment aquatic CO2 losses and the importance of the CO2 evasion component to carbon export via
the aquatic conduit.
Boreal forests are an important ecosystem within high latitude regions containing a globally significant carbon store
in both soils and vegetation (Dunn et al., 2007; Pregitzer and Euskirchen, 2004). The net ecosystem carbon balance (NECB) of individual northern latitude catchments has
shown them to be net sinks for carbon (Dinsmore et al.,
2010, 2013b; Koehler et al., 2011; Nilsson et al., 2008; Olefeldt et al., 2012; Roulet et al., 2007). In boreal forest catchments, carbon export via the aquatic pathway (consisting of
dissolved organic carbon (DOC), dissolved inorganic carbon
(DIC), particulate organic carbon (POC) plus dissolved and
gaseous CO2 and CH4 ) accounted for 4–28 % of carbon uptake via net ecosystem exchange (NEE), representing an im-
Published by Copernicus Publications on behalf of the European Geosciences Union.
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
portant, but spatially and temporally variable component of
the NECB (Öquist et al., 2014; Wallin et al., 2013).
Headwater streams are generally supersaturated in CO2
with respect to the atmosphere, resulting in export via the
aquatic conduit consisting of both the downstream lateral export and vertical evasion of CO2 from the stream surface
(Kling et al., 1991). CO2 evasion has been shown to account
for 13–53 % of the total aquatic flux across a range of northern latitude headwater streams (Billett et al., 2004; Öquist
et al., 2009; Wallin et al., 2013), representing an important
component of the catchment NECB. Low order streams have
been observed to have disproportionately high evasion rates
(Aufdenkampe et al., 2011; Butman and Raymond, 2011;
Raymond et al., 2013). Across Sweden, CO2 evasion from
first order streams was estimated at 0.205 Tg C yr−1 or 39 %
of the total from all streams, despite accounting for only 13 %
of the total stream area (Humborg et al., 2010). Quantifying evasion involves combining dissolved CO2 concentrations and the gas transfer coefficient (KCO2 ) (Hope et al.,
2001). Due to the limited numbers of direct measurements of
the gas transfer coefficient (KCO2 ) (Raymond et al., 2013;
Wallin et al., 2011) and the considerable spatial (Wallin et al.,
2014) and temporal (Crawford et al., 2013; Dinsmore et al.,
2013a) variability in dissolved CO2 concentrations observed
across a wide range of northern latitude catchments, evasion,
and the drivers of this flux are likely to be poorly quantified.
To improve understanding of both lateral downstream export
and vertical evasion of CO2 from headwater streams the concentrations and sources of dissolved CO2 need to be better
To better understand the drivers of stream water CO2 dynamics an increasing number of studies have made continuous, direct measurements of dissolved CO2 concentrations in
stream waters (using in situ, non-dispersive infra-red (NDIR)
CO2 sensors) giving new insights into diurnal, storm event
and seasonal CO2 dynamics (Dinsmore and Billett, 2008;
Dinsmore et al., 2013a; Dyson et al., 2011; Johnson et al.,
2006, 2010). Much of the excess CO2 in temperate and boreal streams originates from terrestrial areas through lateral
subsurface transport through the soil (Hope et al., 2004), confirmed by isotope studies in peatland catchments (Garnett
et al., 2012; Leith et al., 2014). The concentration of CO2
in stream water is largely dependent on the concentration
in terrestrial source areas and the hydrological connectivity
between source areas and the stream channel (Vidon et al.,
2010). Despite the apparent importance of soil sources, most
studies of stream CO2 dynamics take the observed response
in the stream and link it to terrestrial processes without direct
measurements in soils. Quantifying the rate of export of carbon from soil to stream will enable better estimates of CO2
export via the aquatic pathway and in particular contribute to
higher temporal resolution estimates of evasion.
A few studies have made continuous, high frequency CO2
concentration measurements in soils but are largely restricted
to sampling at shallow depths above the water table (DinsBiogeosciences, 12, 1881–1892, 2015
more et al., 2009; Jassal et al., 2004, 2008; Tang et al., 2003).
Higher soil CO2 concentrations have been observed below
the water table and in response to soil re-wetting after storms
(Jassal et al., 2005; Rasilo et al., 2012). In soil, CO2 can
be derived from root respiration, soil organic matter decomposition and weathering of carbonate parent material. Mobilisation of CO2 occurs through the displacement of high
CO2 concentrations in the soil atmosphere, combined with
decreased vertical diffusivity as soil pore space becomes saturated (CO2 diffusion is 10 000 times slower in water than in
air) (Šimůnek and Suarez, 1993). This highlights the importance of considering concentration changes over the full soil
profile depth, especially in horizons which experience frequent water table fluctuations as transient saturation can increase the lateral export of water and dissolved constituents
exponentially (Bishop et al., 2011).
The terrestrial–aquatic interface is a continuum between
the wider catchment area (hillslope), riparian zone and
stream. Riparian zones in headwater catchments commonly
represent a 1–5 m wide area immediately adjacent to the
stream (Luke et al., 2007). Common characteristics of the
riparian zone are higher water table conditions (Burt et al.,
2002), organic rich soils and distinct vegetation composition in comparison to the surrounding hillslope (Lyon et al.,
2011). Hence, the riparian zone has been widely shown to
be a key area for hydrological, ecological and biochemical processes (Naiman and Décamps, 1997; Vidon et al.,
2010). Subsurface hydrological flow paths converge at the
terrestrial–aquatic interface resulting in the riparian zone being the last biogeochemical environment encountered by subsurface water (Bishop et al., 1990; McClain et al., 2003). The
riparian zone has been shown to have a stronger hydrological connection to the stream in comparison to groundwater
or soil water (Buttle et al., 2004; Stieglitz et al., 2003) and
to control stream water chemistry during hydrological events
(Bishop et al., 2004). Water and solute transport has been
shown to be episodic with hotspots and hot moments in export (McClain et al., 2003; Vidon et al., 2010), hence the
full hillslope–riparian–stream continuum needs to be considered at high temporal resolutions to quantify solute transport
across the terrestrial–aquatic interface.
Across five different northern latitude catchments, stream
water CO2 concentrations were shown to be strongly linked
to discharge, with the highest 30 % of flow having the greatest impact on lateral CO2 export (Dinsmore et al., 2013a).
DIC export via the aquatic pathway over a 13-year period in
a headwater Swedish catchment (the same catchment as this
study) was positively correlated with precipitation with export varying from 2.3 to 6.9 g DIC-C m−2 yr−1 (Öquist et al.,
2014). In boreal systems, water and carbon cycles are highly
seasonal and strongly linked to the length of the snow covered period in winter and the spring snow melt event. In these
systems, snowmelt is the major hydrological event of the
year, with one example being the Krycklan catchment where
40–60 % of the annual runoff occurs during only 10–15 %
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
of the year (Laudon et al., 2011). Snowmelt has been shown
to be an important period for carbon export via the aquatic
pathway both as DOC (Laudon et al., 2004; Nilsson et al.,
2008) and gaseous species (Dyson et al., 2011). Another example of the short-term variability in stream water dissolved
CO2 concentrations is the observation in some catchments
of bimodal frequency distributions during storms, indicative
of two distinct CO2 sources within the catchment (Dinsmore et al., 2013a). Combining high resolution concentration measurements in both the soil and the stream is a powerful way of studying hydrological and seasonal variability
in these catchments. However, previous studies which determined dissolved CO2 concentrations in riparian soils and
streams have used either a limited number of discrete measurements (Hope et al., 2004; Öquist et al., 2009) or did not
determine the variability in the depth of lateral subsurface hydrological flow paths transporting CO2 to the stream (Rasilo
et al., 2012). This is likely to under represent variability in
these dynamic systems, especially for short lived hydrological events such as storms.
This study uses high temporal resolution measurements of
CO2 concentrations along a hillslope–riparian–stream continuum in a hydrologically well defined catchment with extensive previous work delineating the hydrological flow paths
along the continuum (Bishop et al., 2004; Seibert et al., 2009;
Stähli et al., 2001). This allows the sources of stream water
CO2 to be investigated while estimating the export of water and CO2 across the terrestrial–aquatic interface. Due to
the position of riparian zones within the catchment and the
greater potential for CO2 production in the organic rich riparian soils we hypothesise that it is the riparian zone, rather
than the wider catchment area, that is maintaining CO2 export to streams.
Materials and methods
Site description
This study was conducted in the 0.13 km2 forested Västrabäcken subcatchment of the 0.47 km2 Svartberget catchment, which is part of the wider 68 km2 Krycklan catchment (64◦ 140 N,19◦ 460 E), 50 km north-west of Umeå, Sweden (Laudon et al., 2013).
The altitude of the catchment ranges from 235 to
310 m a.s.l. (Bishop et al., 1990). Mean annual temperature
(1980–2008) is 1.7 ◦ C, with the maximum (14.6 ◦ C) and
minimum (−9.6 ◦ C) in July and January, respectively. Mean
annual precipitation (1981–2008) is 612 mm, with approximately 35 % falling as snow (Haei et al., 2010). Snow covers
the ground on average for 171 days from October to May to a
depth of between 43 and 113 cm (1980–2007) (Laudon et al.,
2011). The largest hydrological event during the year is a 46 week long snowmelt period in late April/early May which
contributes between 40 and 60 % of annual runoff (Laudon
et al., 2011). The Västrabäcken is a first order, low pH (range
pH 4.5–5.9) with little variability in the bicarbonate equilibrium system (Wallin et al., 2010).
Drainage channels were deepened and widened in the
1920s to improve forest drainage (Bishop et al., 1990).
Scots pine (Pinus sylvestris) is the dominant tree species
in hillslope areas with some Norway spruce (Picea abies)
nearer to the stream. Understorey vegetation is predominately a mix of Vaccinium myrtillus, Vaccinium vitis-idaea
and grasses (Deschampsia flexuosa), with mosses (Sphagnum spp., Polytrichum commune) and wood horsetail (Equisetum sylvaticum) in the wetter riparian areas.
The bedrock is gneiss overlain by 10–15 m of locally derived glacial till. Soils consist of well-developed iron podzols
comprising a surface 5 cm humus layer overlying a 12 cm
thick sandy bleached E-horizon and a 60 cm thick B-horizon
(Nyberg et al., 2001). The riparian zone formed through the
accumulation of organic matter in low lying areas and extends 4 m on either side of the stream channel. Riparian
soils consist of ∼ 70 cm thick peat transitioning to the underlying till at ∼ 90 cm depth. Soil organic content is considerably higher in the riparian soil (> 80 %) compared to
the hillslope podzols (< 5 %) (Nyberg et al., 2001). Saturated hydraulic conductivity is about one order of magnitude
lower in the riparian zone (6.2 × 10−6 m s−1 ) than the hillslope (5.6×10−5 m s−1 ) (Nyberg et al., 2001) decreasing with
depth (Stähli et al., 2001). Porosity and water retention were
higher in the riparian soil (Nyberg et al., 2001; Stähli et al.,
2001). The riparian zone studied is representative of headwater till catchments across the larger Krycklan catchment
(Grabs et al., 2012).
Field methods
Sampling was carried out in both the hillslope podzol soils
(15 m perpendicular to the stream) and riparian zone peat
soils (1.5 m from the stream) (Fig. 1). At each location, two
dipwells with a 10 cm perforated sampling window at either
30–40 or 60–70 cm depth were installed, with the dipwells
separating the soil water from the surrounding soil allowing the measurement of soil water CO2 concentrations. Dipwells were constructed from 50 mm inner diameter (ID) pipe
open at the bottom and sealed at the surface using rubber
bungs (Saint Gobain Performance Plastics, France). To prevent damage to the dipwells over winter due to freezing, the
short above ground section was covered in insulation foam.
Each dipwell contained a Vaisala CARBOCAP GMP221
non-dispersive infra-red (NDIR) CO2 sensor (range 0–5 %).
Prior to deployment, sensors were enclosed in a water-tight,
gas-permeable membrane (Johnson et al., 2006, 2010). At
each sampling point a third dipwell, constructed from 90 mm
ID pipe perforated along its entire 1 m length and open
at the bottom, was used for the measurement of water table depth (Level Troll 300, In-situ, USA) and soil temperature (CS457A, Campbell Scientific, USA), with sensors susBiogeosciences, 12, 1881–1892, 2015
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
concentrations are given in units of ppmv with mg CO2 C L−1 used for export calculations.
The export of water and CO2 from each m2 of hillslope
Riparian zone
50 cm
and riparian zone were estimated using a flow-concentration
model, a similar approach to the riparian integration model
concept used previously in this, and similar, hillslope–
100 cm
riparian systems (Grabs et al., 2012; Seibert et al., 2009). Our
study used measured water table positions in the hillslope
and riparian zone while previous studies have used a correla150 cm
tion between groundwater and runoff dynamics (Grabs et al.,
The model was constructed by subdividing the 90 cm deep
10 m
20 m
soil profile into 5 cm horizons. The daily lateral water export
Figure 1. Schematic of the hillslope–riparian transect used in this
from each 5 cm soil layer was estimated by combining the
Figure study.
1. Schematic
the hillslope-riparian
in this study.
were installed
2 sensorswinCO2ofsensors
were installed
with CO
measured volumetric water content with lateral saturated hyin dipwells with sampling windows at 30-40 cm and 60-70 cm depth. An additional dipwell for water
at 30–40 and
for watable and
soil temperature
was 60–70
the full
to 90 cm
depth. Grey
box indicates
draulic conductivity estimated by Stähli et al. (2001). Total
soil temperature
the fullover
the zone
(between max
min water table
the hydrological
daily water export from the full soil profile was estimated by
year. 90 cm depth. The grey box indicates the zone of transient saturation
adding together the lateral flow from all 5 cm horizons below
(between max and min water table positions) over the hydrological
the daily mean water table. As CO2 concentration was only
measured at two depths (30–40 and 60–70 cm) these were assumed to represent the concentration above and below 45 cm
depth. Daily average CO2 concentrations below the water tapended at 90 cm depth. All sensors were connected to a data
ble, in mg CO2 -C L−1 , were multiplied by the water export,
logger (CR1000, Campbell Scientific, USA) which recorded
with CO2 export expressed in units of mg CO2 -C m−2 day−1 .
measurements at 30 min intervals. The system ran for three
The model was run twice to estimate the export from (1) podmonths (from 1 July 2012) to allow it to become stabilised
zol hillslope soils and (2) riparian organic soils. Hillslope exand equilibrated before the main measurement period over
port was taken to represent the input of water and CO2 into
one hydrological year (1 October 2012–30 September 2013).
the riparian zone with riparian export representing the total
Soil moisture content was recorded by time domain reflecterrestrial water and CO2 export to the stream.
tometry (TDR) probes (Laudon et al., 2013; Nyberg et al.,
Downstream CO2 export was determined by multiplying
mean stream water CO2 concentration with discharge at
Measurements were made at one point in the hillslope and
outlet. Based on the assumption that all stream
riparian zone assuming, as with all transect studies, that these
derived from terrestrial inputs, evasion was
points are representative of the catchment overall. A single
the downstream lateral CO2 export
transect approach is appropriate for this catchment due to the
the riparian zone to the stream.
small area (0.13 km ) and limited variability in soils and vegWhilst
may not be completely
etation (Bishop et al., 1990; Lyon et al., 2011). Furthermore,
total inputs and thereoverland flow or deeper groundwater inputs to the stream
catchment is likely
have been shown to be limited in this catchment (Laudon
pH and short waet al., 2004; Peralta-Tapia et al., 2014). The sampled riparter
results therefore
ian zone is consistent with the 13 riparian zones in the wider
variability in a
Krycklan Riparian Observatory (Grabs et al., 2012).
temporal resoStream water dissolved CO2 concentrations (measured uslution.
ing the same Vaisala CO2 sensors), plus discharge were mea0 cm
sured continuously in a heated dam house at the catchment
outlet (Laudon et al., 2013), 200 m downstream from and
over the same time period as soil measurements.
Data processing and analysis
CO2 sensor output was corrected for temperature and pressure using the method of Tang et al. (2003) but using algorithms supplied by the manufacturer specific to the GMP221
sensors. In addition to atmospheric pressure, the correction
also accounted for the head of water above the sensor, related
to the water table depth at the time of sampling. Corrected
Biogeosciences, 12, 1881–1892, 2015
Hillslope–riparian hydrological connectivity
Water table was significantly higher in the riparian zone compared to the hillslope, but with similar temporal variability
(Fig. 2). Mean (±SD) hillslope water table during the hydrological year was −63 ± 16 cm compared with −37 ± 10 cm
in the riparian zone (Table 1). Volumetric soil moisture
content was also higher in the riparian zone (mean ±SD
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
Table 1. Mean (min–max) calculated from all continuous measurements over the hydrological year (1 October 2012–30 September
2013) in the hillslope and riparian zone. Significant differences between the hillslope and riparian zone sampling points (at P < 0.01)
are indicated by ∗∗ .
(−118 to −10)
(−83 to −12)
CO2 (ppmv) 30–40 cm∗∗
(1680–11 730)
15 130
(4430–21 730)
CO2 (ppmv) 60–70 cm∗∗
(1170–15 770)
23 100
(16 140–31 920)
Water table depth (cm)∗∗
Temperature (◦ C)∗∗
Annual mean (±SD) soil temperature was 4.1 ± 3.0 and
3.9 ± 3.0 ◦ C in the hillslope and riparian zone respectively
with a strong seasonal trend (Fig. 2). Soil temperature (measured at 70 cm depth) did not fall below freezing, with minimum temperatures in the hillslope (0.8 ◦ C) and riparian
(0.7 ◦ C) (Table 1) reached at the beginning of May (Fig. 2).
Water table was above the 60 cm CO2 measurement depth
for 32 and 97 % of the time and above the 30 cm depth 7
and 59 % of the time, in the hillslope and riparian zone, respectively. These values describe the relative proportion of
the measurement period over which the CO2 sensors were
Hillslope–riparian CO2 concentrations
CO2 concentration (ppmv)
WT position (cm)
Soil Temperature (°C)
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
CO2 concentrations were on average higher in the riparian
zone than the corresponding depth in the hillslope (Table 1).
The highest mean (±SD) CO2 concentrations were at 60–
70 cm in the riparian zone (23 100 ± 4100 ppmv) with the
40000 a.
Riparian 30 cm
Riparian 60 cm
Hillslope 30 cm
Hillslope 60 cm
lowest at 30–40 cm in the hillslope (4410 ± 2570 ppmv). At
both depths the hillslope and riparian zone CO2 concentrations were significantly different (Table 1).
There was considerable temporal variability in CO2 con10000
centration in the hillslope, which displayed a baseline and
peak pattern (Fig. 2). Baseline concentrations were gener0
ally < 2000 ppmv. In the winter period (January–March), median concentrations at 60–70 cm depth (1600 ppmv) were
0 b.
24 % less than those at 30–40 cm depth (2110 ppmv). During drier periods in summer (July–September), the base−30
line was higher with median concentrations at 60–70 cm
(1770 ppmv), 61 % less than those at 30–40 cm depth
(4530 ppmv). The range was also higher in summer (7640
and 6470 ppmv at 30–40 and 60–70 cm depths) than in winRiparian zone
ter (680 and 870 ppmv).
Periodically, sharp increases in hillslope CO2 concentra10.0 c.
tions were observed, corresponding to a rise in water table position (Fig. 2). CO2 concentrations in the hillslope
had a positive correlation with hillslope water table at 30–
40 cm ((r 2 = 0.43; P < 0.001) and 60–70 cm (r 2 = 0.65;
P < 0.001) depths. Over the measurement period the two
spikes with the highest CO2 concentrations (14 100 and
Riparian zone
13 160 ppmv) occurred when the water table was above the
Oct 2012
Jan 2013
Apr 2013
Jul 2013
Oct 2013
level of the deeper (60–70 cm depth) sensor at −41 and
−51 cm, respectively.
CO2 concentrations in the riparian zone did not display
Figure 2. Time series of hillslope and riparian zone (a) CO2 con2. Time seriescentrations
of hillslope
a. cm
at 30-40
cm and peak pattern as the hillslope. At 60–
2 concentrations
the same
30–40 and
(b) water tablesampled
70 cm depths,and
b. (c)
table and across
c. soilthe
the Horifull measurement
soil temperature
full measurement
70 cm depth,
CO2 concentrations peaked at 31 920 ppmv in
al lines in b. zontal
the 30-40
60-70 cm
in (b)
the deepest
by the cm
30– and October
falling to 16 140 ppmv in April (Fig. 2). This
40 and 60–70 cm depth CO2 sensors.
change in concentration closely followed soil temperature
which decreased over a similar period from 9.2 ◦ C to the annual minimum of 0.7 ◦ C on 26 April 2013 (Fig. 2), coincid30
of 0.73 ± 0.008 and 0.46 ± 0.004 m m at 30 and 60 cm
ing with minimum CO2 concentrations. CO2 concentrations
depths, respectively) compared to the hillslope (0.50 ± 0.01
had a weak positive correlation with riparian water table at
and 0.45 ± 0.003 m3 m−3 at 30 and 60 cm depths, respec30–40 cm (r 2 = 0.12; P < 0.001) and a weak negative cortively).
relation at 60–70 cm (r 2 = −0.21; P < 0.001) depth. At 30–
Biogeosciences, 12, 1881–1892, 2015
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
Flow m3 m−2
CO2−C m−2
export was highest in May (49 mm month−1 ) due to dominant flow at 25–50 cm depth with maximum monthly flow
(9 mm month−1 ) at 35–40 cm depth.
Total annual CO2 export from the riparian zone to the
stream channel over the hydrological year was estimated
at 3008 mg CO2 -C m−2 yr−1 . Two monthly peaks in riparian CO2 export to the stream occurred over the hydrological year, in December (529 mg CO2 -C m−2 month−1 ) and
May (522 mg CO2 -C m−2 month−1 ). CO2 export was highest in a narrow zone between 25 and 50 cm depth (Fig. 3).
In December, water flow (5 mm month−1 ) and CO2 export
(77 mg CO2 -C m−2 month−1 ) were highest from 35–40 cm
depth. In May, maximum flow of both water and CO2
was from the same depth (35–40 cm) at 9 mm month−1 and
96 mg CO2 -C m−2 month−1 .
Downstream CO2 export and evasion
Total catchment runoff over the hydrological year was
265 mm yr−1 (Fig. 4). Total downstream lateral CO2 exFigure 3. Total monthly export of CO2 from the (a) hillslope and
port from the catchment was estimated at 1183 mg CO2 Figure 3. Total
monthly export
of CO2 from
the a. hillslope and
b. riparian zone across the measure(b) riparian
zone across
the measurement
C m−2 yr−1 . There was considerable temporal variability in
ment period
the downstream lateral export of CO2 from the catchment,
related to temporal variability in discharge (Fig. 4). Median
40 cm depth in the riparian) depth. At 30–40 cm depth in the
downstream lateral export was 1.2 mg CO2 -C m−2 day−1
riparian zone there was considerably more temporal variabilwith two large spikes (57 and 35 mg CO2 -C m−2 day−1 ) cority in CO2 concentrations than at 60–70 cm depth with conresponding to sudden increases in discharge after storm
centrations at 30–40 cm depth rising during periods of higher
events. Over the same period the input of CO2 from the soil
water table. Minimum concentrations (4430 ppmv) occurred
to the stream was 3008 mg CO2 -C m−2 yr−1 . Based on the asin April before the spring snowmelt event.
sumption that all stream water CO2 is derived from soil input
then by subtraction 1825 mg CO2 -C m−2 yr−1 is lost between
3.3 Hillslope–riparian water and CO2 export
the soil and the stream (i.e. evasion from the stream surface),
which accounted for 60 % of export via the aquatic pathway
Annual water export from each m2 of hillslope over the hy(Fig. 4).
drological year was estimated at 230 mm yr−1 . Across the
year, water export was consistently low with flow largely restricted to below ∼ 50 cm depth. Total monthly export was
4 Discussion
highest in May (109 mm month−1 ) accounting for 47 % of
the annual hillslope water export, due to greater flow at 25–
4.1 Hillslope–riparian CO2 concentrations
35 cm depth (37 mm month−1 ). Hillslope CO2 export over
It was hypothesised that CO2 concentrations would be higher
the hydrological year was 1144 mg CO2 -C m−2 yr−1 and folin the riparian zone as a result of enhanced production (by delowed a similar pattern to water export. CO2 export was con−1
composition of soil organic matter and root respiration) due
siderably higher in May (482 mg CO2 -C m month ), acto the higher organic matter content of riparian peat (> 80 %)
counting for 42 % of the annual CO2 export, largely due to
compared to hillslope podzols (< 5 %) (Nyberg et al., 2001)
considerable export (93 mg CO2 -C m month ) from 45–
and greater mobilisation of CO2 due to the generally wet50 cm depth (Fig. 3).
ter conditions found in riparian zones (Burt et al., 2002).
Changes in the soil water content can result in: (1) initial
3.4 Riparian–stream water and CO2 export
displacement of high CO2 concentration soil atmosphere as
Annual water export from the riparian zone to the stream
soils become saturated and (2) decreased vertical diffusion as
channel over the hydrological year was estimated at
soil pore space becomes saturated with water. Weathering of
270 mm yr−1 . Below 65 cm depth, although water export was
carbonate parent material can also contribute but carbonate
relatively low (< 3 mm month−1 ) it occurred for > 97 % of the
bedrock is not found in this catchment (Wallin et al., 2013).
measurement period. Water export was highest in a zone beMean hillslope concentrations (Table 1) are within the range
tween 25 and 50 cm depth. At shallower depths (0–25 cm)
(∼ 400 to ∼ 10 000 ppmv) reported by studies conducted in
flow was restricted to the wettest months. Total monthly
similar forest podzol soils (Jassal et al., 2004, 2005; Tang
Biogeosciences, 12, 1881–1892, 2015
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
ture, which had a strong seasonal cycle (Fig. 2) suggesting
root respiration and/or organic matter decomposition, driven
by changes in soil temperature, provides an additional con6
trol on CO2 concentrations. During periods of generally high
water tables in the riparian zone, such as during the spring
snowmelt event in May, concentrations increased (Fig. 2)
again highlighting the importance of water table fluctuations
on CO2 concentrations in soils.
Overall, the large temporal variability in porewater CO2
concentration, especially in the hillslope, can largely be attributed to water table fluctuations altering the rate of CO2
mobilisation and vertical diffusion of CO2 stored in soil
pore space. This confirms the results of others who measured higher CO2 concentrations during periods of water tac.
ble fluctuation (Jassal et al., 2005; Rasilo et al., 2012). The
importance of the water table for CO2 concentration dynamics suggests that pore water CO2 concentration differences
between the hillslope and the riparian zone are strongly influ0
enced by the altered diffusion gradient and surface exchange
of CO2 with the atmosphere; the riparian zone had continud.
ally higher water tables than the hillslope (Fig. 2). However,
despite the importance of water table fluctuations, most of
the existing studies involving continuous use of CO2 sensors
have been focused predominately above the water table, po0
Oct 2012
Jan 2013
Apr 2013
Jul 2013
Oct 2013
tentially missing this observation (Jassal et al., 2004, 2005;
Tang et al., 2003). This highlights the importance of considering CO2 concentration changes over the full soil proFigure 4. Mean daily (a) discharge (Q), (b) CO2 export from the
4. Mean daily a.
b. CO2(c)export
from the
to the stream,
c. especially
zone to(Q),
the stream,
CO2 export
file depth,
in horizons which experience frequent
ateral CO2 export
and d.CO
over the
hydrological year. fluctuations between wet and dry conditions. In this study,
(d) vertical
the hydrological
2 evasion
2 evasion
CO2 concentrations were only determined at two sampling
depths (30–40 and 60–70 cm). In the riparian zone the shalet al., 2003). Mean riparian concentrations at 30–40 cm depth
lower measurement depth was below the water table 59 % of
32 riparian zone at the same
were similar to those in a Finnish
the time giving a range of measurements under saturated and
depth (14 200–16 500 ppmv) (Rasilo et al., 2012) but CO2
unsaturated conditions. The deeper measurement point was
concentrations at 60–70 cm depth were considerably higher
below the water table 97 % of the time giving concentrations
(Table 1).
under saturated conditions. These two sampling depths thereThere was considerable temporal variability in CO2 confore gave a good overall representation of soil conditions, and
centrations, especially in the hillslope, corresponding to watheir respective CO2 concentrations.
ter table fluctuations (Fig. 2). The CO2 concentrations in
the hillslope podzol soils over the measurement period, with
4.2 Hillslope–riparian–stream water and CO2 export
peaks of 14 100 and 13 160 ppmv, both occurred during high
water table conditions, as also observed by Rasilo et al.
The total export of water per m2 of hillslope was
(2012). During the winter months (January–April) when the
230 mm yr−1 and from the riparian zone 270 mm yr−1 . Both
water table was generally low, CO2 concentrations remained
estimates correspond well (proportional to the relative uprelatively stable at ∼ 2000 ppmv (Fig. 2) suggesting that the
slope contributing area) to the annual catchment runoff
vertical exchange of CO2 is greater than the lateral export
of 265 mm yr−1 . The lateral movement of water along the
during these periods (Dyson et al., 2011).
hillslope–riparian–stream continuum is dependent on a numTemporal variability in riparian zone CO2 concentrations
ber of assumptions, principally that lateral subsurface flow
was more pronounced at 30–40 cm depth which can largely
is the dominant hydrological pathway and that these flow
be attributed to water table fluctuations (the sensor was bepaths are perpendicular to the stream. These assumptions
low the water table for 59 % of the measurement period). The
have been validated for the study catchment by the observed
60–70 cm sampling depth was below the water table for 97 %
planar nature of the water table (Cory et al., 2007) and
of the measurement period with water table variability alone
through stable isotope studies (Laudon et al., 2004; Peraltaunlikely to explain the observed pattern in riparian CO2 conTapia et al., 2014). Alternative flow paths, such as overland
centrations. CO2 concentration was linked to soil temperaflow and groundwater recharge are not important at this site
(mg CO2−C m−2 day−1)
Riparian Export
Downstream Export
(mg CO2−C m−2 day−1) (mg CO2−C m−2 day−1)
(L day−1)
12 a.
Biogeosciences, 12, 1881–1892, 2015
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
(Grabs et al., 2012). Downslope lateral water flow occurs in
saturated soils, with minimal lateral flow occurring above the
water table, in accordance with the transmissivity feedback
conceptualisation of subsurface flows in shallow till catchments (Bishop et al., 2011; Rodhe, 1989).
Transmissivity feedback, which has been widely observed
in this catchment, is defined as the increase in lateral saturated hydraulic conductivity towards the soil surface, resulting in more lateral flow as water table rises into near surface soil horizons (Bishop et al., 1990, 2004; Laudon et al.,
2004). The drier hillslope podzol soils had a greater potential
to store water without increasing water table (Seibert et al.,
2003), limiting the time in which high water table conditions
occurred; mean water position in the hillslope was −63 cm.
Water table only moved into higher lateral hydraulic conductivity horizons during the spring snowmelt event in May
when 42 % of annual hillslope CO2 export occurred, emphasising the importance of the spring snowmelt event in this
system. The results from this study show that maximum water export occurs where water table position and soil hydrological properties (lateral hydraulic conductivity and soil water content) combine to produce the set of conditions most
conducive to water flow, in agreement with the transmissivity feedback principle.
The concept of a riparian chemosphere has been conceptualised by and shown to exist in the catchment studied here
(Bishop et al., 2004; Seibert et al., 2009), with the chemistry
of water flowing laterally through the catchment determined
by soil interactions. The final soil encountered by subsurface
waters (in this study the riparian peat soils) will determine
the composition of water transported across the terrestrial–
aquatic interface (Bishop et al., 2004). Over the hydrological
year, total CO2 export per m2 of hillslope was 1144 mg CO2 C m−2 yr−1 with export from the riparian zone estimated at
3008 mg CO2 -C m−2 yr−1 . The organic rich peat soils of the
riparian zone therefore represent a significantly greater CO2
export source than the podzol hillslope soils, despite accounting for < 10 % of the catchment area. This can be related to
the higher measured CO2 concentrations (Fig. 2). The importance of riparian–stream CO2 export is in agreement with the
results for other catchments (Peter et al., 2014; Rasilo et al.,
2012) and for total organic carbon in this catchment (Grabs
et al., 2012).
Few studies have estimated carbon export across the
terrestrial–aquatic interface, especially for CO2 . The total
annual export of CO2 in this study (3.0 g CO2 -C m−2 yr−1 )
was similar, given the large inter-annual variability, to
the estimate for DIC (3.2 g DIC m−2 yr−1 ) produced from
spot measurements (Öquist et al., 2009) and the 2.3–
6.9 g DIC m−2 yr−1 estimated over a longer period (1997–
2009) in the same catchment (Öquist et al., 2014). As expected, CO2 export was lower than in peat dominated catchments in Sweden (3.1–6.0 g CO2 -C m−2 yr−1 ) (Nilsson et al.,
2008) and Scotland (11.2–15.5 g CO2 -C m−2 yr−1 ) (Dinsmore et al., 2010, 2013b).
Biogeosciences, 12, 1881–1892, 2015
The results of this study suggest that the riparian zone, and
not the wider hillslope, is the dominant source of CO2 entering the stream but the contribution of the riparian zone to water and CO2 transport has been shown to be episodic, resulting in hotspots and hot moments when export is greatest (McClain et al., 2003; Vidon et al., 2010). CO2 export was highest in a narrow band between 30 and 50 cm depth, which accounted for 71 % of CO2 export. The results from this study
suggest that the riparian zone contains two distinct sources
of CO2 export; high export rates at 30–50 cm depth as water table moves into more superficial horizons and a deeper
(> 65 cm depth) continuous but smaller export (Fig. 3). The
presence of these two water sources in this catchment has
also been shown isotopically (Peralta-Tapia et al., 2014) and
indicated by the observed bimodal frequency distributions in
stream water CO2 concentrations during storm events (Dinsmore et al., 2013a). Therefore, the riparian export rates of
water and CO2 measured here can be used to explain the observed changes in stream water CO2 concentrations. This approach can be used as the catchment chosen for this study is
relatively simple in terms of the water flow paths (with water transported laterally through the soil at < 1 m depth with
groundwater and overland flow not significant) and the consistency of the riparian lateral extent down the stream reach.
In catchments with more complex hydrology or where the
riparian lateral extent is variable, riparian CO2 export alone
may not account for all variability in stream water CO2 dynamics and additional sources would need to be considered.
Peaks in water and CO2 export occurred in late autumn
(October–December) and May. The export of water from the
riparian zone was highest during May at 49 mm month−1
when the spring snowmelt event occurred, accounting for
18 % of the annual water export from the riparian zone. In
May, at the onset of spring snowmelt, CO2 concentrations
were close to minimum values of 3840 and 14 400 ppmv
at 30–40 and 60–70 cm depths in the riparian zone, resulting in relatively low CO2 export despite large water export at this time. In October–December, despite only moderate water export, CO2 export was high (275–529 mg CO2 C m−2 month−1 ) (Fig. 3). During these months, CO2 concentrations in the riparian soil were at or close to their maximum values (Fig. 2), coinciding with maximum soil water DOC concentrations in late summer/autumn observed in
the catchment (Lyon et al., 2011). This observed pattern of
peaks in CO2 export suggests that riparian export is a function of both season and runoff. This highlights the importance of capturing hydrological extremes when quantifying
annual estimates of downstream export and evasion of CO2
across catchments and scales.
The lateral export of riparian dissolved CO2 is therefore
the main source of stream water CO2 , with the amount of
CO2 exported related to the depth of water flow through the
riparian zone. The riparian export of CO2 can therefore be
used as an estimate of the catchment CO2 export to partition
the export of CO2 via the aquatic conduit.
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
Downstream CO2 export and evasion
Total downstream lateral CO2 export, calculated from the
CO2 concentration and discharge at the catchment outlet, over the hydrological year was 1.2 g CO2 -C m−2 yr−1 .
This is within the range 0.9–1.3 g CO2 -C m−2 yr−1 of estimates for the same catchment (Öquist et al., 2009) and a
peatland catchment in southern Scotland (Dinsmore et al.,
2010, 2013b). Due to the greater range in discharge (0.02–
15.3 L s−1 ) than dissolved CO2 concentrations (daily mean
CO2 concentration ranged from 1.8–7.2 mg CO2 -C L−1 );
discharge had a greater influence on temporal variability in
downstream lateral CO2 export (Fig. 4).
By subtracting downstream lateral CO2 export
(1183 mg CO2 -C m−2 yr−1 ) from the estimate of soil
export over the same period (3008 mg CO2 -C m−2 yr−1 ),
an estimated 1825 mg CO2 -C m−2 yr−1 was released to the
stream but not accounted for by downstream lateral export.
Photosynthetic uptake by aquatic plants or in stream respiration is unlikely to be important in this catchment due to the
short water residence times (typically ∼ 4.5 h) and the low
temperatures (Öquist et al., 2009). Hence the remaining CO2
is likely to be evaded from the stream surface. Our evasion
estimate from the stream surface (1825 mg CO2 -C m−2 yr−1 )
was at the lower end of evasion rates determined from direct
point measurements in the same catchment (Wallin et al.,
2011). However, evasion accounted for 60 % of export via
the aquatic pathway supporting previous findings of the
rapid CO2 loss from the stream (Öquist et al., 2009).
Estimating evasion rate based on this approach is dependent on two assumptions: (1) all stream water CO2 is derived
from lateral soil inputs from the riparian zone and (2) a short
water transport time between the riparian zone and the stream
to allow rapid exchange of water and CO2 during events.
In headwater systems, in-stream productivity (through biological respiration and photo-degradation of DOC) is limited
by the cold temperatures, low pH and short water residence
times (Dawson et al., 2001). Additionally, CO2 evasion from
headwater streams has been widely shown to be composed
predominately of recently fixed, plant derived CO2 transported from the surrounding soil (Billett et al., 2012; Leith
et al., 2014). The transport time for water between the riparian zone and the stream varied with depth from < 1 h
at < 15 cm depth up to ∼ 25 h at > 70 cm depth. The lateral exchange of water and CO2 between the riparian zone
and the stream was rapid enough for the approach to be
valid at the daily scale that was used in the model. Additionally, the total volume of water exported from the riparian zone (270 mm yr−1 ) corresponded well with the annual
runoff from the catchment (265 mm yr−1 ) suggesting that no
other inputs of water are contributing. Riparian CO2 export
to the stream is therefore sustaining the lateral downstream
export and vertical evasion of CO2 .
There was considerable temporal variability in the evasion estimate (Fig. 4) related to variability in CO2 dynamics
in both the terrestrial (Fig. 2) and aquatic systems (Fig. 4).
During the two largest storm events, in which discharge increased suddenly over a very short time period (< 1 day),
downstream lateral export spiked but without a corresponding increase in soil export. Evasion was therefore not estimated during these periods. This also occurred during the
onset of the spring flood. This suggests that during storms
there may be a rapid input of overland flow or direct channel water input contributing to discharge without interacting with the soil. The model used to estimate soil CO2 export did not include overland flow so may be underestimating the total flow of water and CO2 into the stream during
storm events. Overland flow has been shown to be a relatively minor flow path within the Västrabäcken catchment
(Grabs et al., 2012; Peralta-Tapia et al., 2014). However, during the spring flood, up to 20 % of stream runoff was found
to be derived from snowmelt transported via overland flow
(Laudon et al., 2004). Thus during the spring snowmelt period, water and CO2 stored within the snowpack may be an
additional export source, bypassing the soil profile. In two
Finnish catchments, CO2 concentrations in the snow pack
of 500–1900 ppmv were recorded (Dyson et al., 2011) with
snowmelt estimated to contribute 35–46 % of downstream
lateral CO2 export during the spring snowmelt period (Dinsmore et al., 2011). Over the course of the spring snowmelt
event in the same Finnish catchments the isotopic signature
of evaded CO2 showed a decreasing contribution from recently fixed, plant derived CO2 from near surface soil horizons with a corresponding increase in the atmospheric CO2
component, likely derived from the melting snow pack (Billett et al., 2012). In some headwater catchments, the effects
of melt water and overland flow would need to be accounted
for in annual estimates of riparian CO2 export.
Although the annual estimate of riparian export and evasion in the study compare well to estimates from discrete
measurements (Öquist et al., 2009), the results of this study
highlight the importance of high frequency, direct measurements of CO2 concentrations given the high temporal variability in CO2 dynamics in the terrestrial and aquatic systems. Terrestrial processes have been shown to have an important role in determining CO2 export via the aquatic pathway in a wide range of catchments (Abril et al., 2014; Butman and Raymond, 2011; Crawford et al., 2013). The results from this catchment indicate that terrestrial–aquatic export of CO2 was controlled by riparian water table dynamics, highlighting the potential importance of riparian zones in
headwater catchments. Changes in climate, especially greater
variability in precipitation patterns (IPCC, 2007), have the
potential to alter riparian water table dynamics and since carbon export via the aquatic pathway has been shown to be
positively correlated with precipitation (Öquist et al., 2014),
impact on the export of CO2 to streams and the NECB of
boreal headwater catchments.
Biogeosciences, 12, 1881–1892, 2015
F. I. Leith et al.: Carbon dioxide transport in a boreal headwater catchment
CO2 concentrations were significantly higher in the riparian zone than hillslope soils, which we infer was due to
(1) greater production of CO2 in riparian peats compared
to the hillslope podzols and (2) higher water table positions
limiting the vertical CO2 diffusion and exchange with the atmosphere. The results of this study suggest that the riparian
zone, and not the wider hillslope, is the dominant source of
CO2 entering the stream with a hotspot for export observed
at 30–50 cm depth (accounting for 71 % of total riparian export). Seasonal variability was high with peaks in export during the spring flood and autumn storm events highlighting
the importance of high frequency measurements in this very
dynamic system.
Downstream CO2 export (determined from stream water
dissolved CO2 concentrations and discharge) was 1.2 g CO2 C m−2 yr−1 . Subtracting downstream lateral export from
riparian input (3.0 g CO2 -C m−2 yr−1 ) gives 1.8 g CO2 C m−2 yr−1 which can be attributed to evasion losses (accounting for 60 % of export via the aquatic pathway). The
results highlight the importance of terrestrial CO2 export, especially from the riparian zone, for determining catchment
aquatic CO2 losses and especially for maintaining the high
evasion fluxes from boreal headwater streams.
Acknowledgements. The work was funded by a UK Natural Environment Research Council (NERC) Algorithm PhD studentship
(NE/I527996/1) and the Swedish Research Council (2012-3919).
We thank all those involved in the Krycklan catchment study,
especially Peder Blomkvist and Victor Sjöblom, for all their field
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