Published as: O’ Flynn, C.J., Healy, M.G., Lanigan, G.J., Troy,... O. 2013. Impact of chemically amended pig slurry on greenhouse...

Published as: O’ Flynn, C.J., Healy, M.G., Lanigan, G.J., Troy,... O. 2013. Impact of chemically amended pig slurry on greenhouse...
Published as: O’ Flynn, C.J., Healy, M.G., Lanigan, G.J., Troy, S.M., Somers, C., Fenton,
O. 2013. Impact of chemically amended pig slurry on greenhouse gas emissions, soil
properties and leachate. Journal of Environmental Management 128: 690-698.
http://dx.doi.org/10.1016/j.jenvman.2013.06.020
Impact of chemically amended pig slurry on greenhouse
gas emissions, soil properties and leachate
Cornelius J. O’ Flynna, Mark G. Healy a*, Gary J. Laniganb, Shane M. Troyc, Cathal Somersb,
Owen Fentonb
a
Civil Engineering, National University of Ireland, Galway, Co. Galway, Ireland.
b
Teagasc, Environmental Research Centre, Johnstown Castle, Co. Wexford, Ireland.
c
Scottish Rural College, Roslin Institute Building, Edinburgh, UK.
*Corresponding author. Tel: +353 91 495364; fax: +353 91 494507. E-mail address:
[email protected]
Abstract
The effectiveness of chemical amendment of pig slurry to ameliorate phosphorus (P) losses in runoff
is well studied, but research mainly has concentrated only on the runoff pathway. The aims of this
study were to investigate changes to leachate nutrient losses, soil properties and greenhouse
gas (GHG) emissions due to the chemical amendment of pig slurry spread at 19 kg total
phosphorus (TP), 90 kg total nitrogen (TN), and 180 kg total carbon (TC) ha-1. The
amendments examined were: (1) commercial grade liquid alum (8% Al2O3) applied at a rate
of 0.88:1 [Al:TP] (2) commercial-grade liquid ferric chloride (38% FeCl3) applied at a rate of
1 0.89:1 [Fe:TP] and (3) commercial-grade liquid poly-aluminium chloride (PAC) (10% Al2O3)
applied at a rate of 0.72:1 [Al:TP]. Columns filled with sieved soil were incubated for 8 mo at
10oC and were leached with 160 ml (19 mm) distilled water wk-1. All amendments reduced the
Morgan’s phosphorus and water extractable P content of the soil to that of the soil-only
treatment, indicating that they have the ability to reduce P loss in leachate following slurry
application. There were no significant differences between treatments for nitrogen (N) or
carbon (C) in leachate or soil, indicating no deleterious impact on reactive N emissions or soil
C cycling. Chemical amendment posed no significant change to GHG emissions from pig
slurry, and in the cases of alum and PAC, reduced cumulative N2O and CO2 losses. Chemical
amendment of land applied pig slurry can reduce P in runoff without any negative impact on
nutrient leaching and GHG emissions. Future work must be conducted to ascertain if more
significant reductions in GHG emissions are possible with chemical amendments.
Keywords: pig slurry; P sorbing amendments; Water Framework Directive; nitrate
1. Introduction
The European Union Water Framework Directive (EU WFD) (European Commission (EC),
2000) aims to achieve ‘at least’ good ecological status for all water bodies, including rivers,
lakes, groundwater, estuaries and coastal waters, in all member states by 2015. To meet this
objective, Programmes of Measures (POM) must be implemented in all EU member states. In
Ireland, POM are enacted by the Nitrates Directive (European Economic Community, 1991),
which, amongst other measures, limits the magnitude, timing and placement of inorganic
fertilizer and organic manure applications to land.
2 In Ireland, as part of the National Action Programme (NAP) to address the requirements of
the EU WFD, the maximum amount of livestock manure that may be spread on land, together
with manure deposited by the livestock, cannot exceed 170 kg nitrogen (N) ha-1 yr-1 and 49
kg phosphorus (P) ha-1 yr-1. This limit is dependent on grassland stocking rate and soil test
phosphorus (STP; based on plant available Morgan’s P (Pm)). Soil P Index categories of 1
(deficient) to 4 (excessive) are used to classify STP concentrations in Ireland (Schulte et al.,
2010). Phosphorus losses from P Index 4 soils have the potential to become exported along
the transfer continuum within a catchment, and may adversely affect surface and groundwater
quality (Wall et al., 2011). The amount by which these limits can be exceeded will be
reduced gradually to zero by January 1, 2017. These new regulations will have an impact on
the pig industry in particular, as it is focused in relatively small areas of Ireland, and will, in
effect, reduce the amount of land available for the application of pig slurry. This may lead to
the need for pig slurry export, which is energetically questionable at distances over 50 km
(Fealy and Schroder, 2008).
Landspreading is currently the most cost effective treatment option for pig slurry in Ireland
(Nolan et al., 2012). Due to the high concentrations of pig farming in certain areas, in the
midlands and south of the country especially, the constant application of pig slurry results in
certain fields (those nearest the farm or the most suitable areas for spreading (Wall et al.,
2011)) becoming high in STP, which may take years-to-decades to be reduced to
agronomically optimum levels (Schulte et al., 2010).
When applications of pig slurry are followed by rainfall events, incidental (short-term),
diffuse transfers of P and N may occur in runoff. Losses of both P and N may also occur
through leaching, which ultimately could have adverse consequences for water bodies
3 (McDowell and Sharpley, 2001; Fenton et al., 2011; Sophocleous, 2011). Karstified aquifers,
which are overlain by free-draining soils, are particularly susceptible to groundwater
pollution, as they have less attenuation potential than surface runoff pathways and there is a
high potential for macropore flow of dissolved and particulate forms of P (Kramers et al.,
2012). In Ireland, karstified limestone covers approximately 20% of the area of the country
(Daly, 2005), and much pig farming is conducted in karst-covered areas.
Chemical amendment of pig slurry has been shown to be an effective means of reducing
surface runoff of P and suspended sediment (SS) by numerous researchers (Smith et al.,
2001, 2004; Dou et al, 2003), but as yet, the role pig slurry amendments have to play in
controlling leached losses has not been investigated. O’ Flynn et al. (2012a, 2012b) examined
the effectiveness and feasibility of different chemical amendments, added to pig slurry, in
reducing P, SS and metal concentrations in a series of laboratory studies, conducted first at
bench scale (O’ Flynn et al., 2012a) and then using a laboratory rainfall simulator (O’ Flynn
et al., 2012b). In the latter study, O’ Flynn et al. (2012b), found additions of alum, ferric
chloride (FeCl3) and poly-aluminium chloride (PAC) reduced total phosphorus (TP) and SS
losses in surface runoff, without posing a significant risk of metal losses.
Although there has been much work done on the chemical amendment of surface applied pig
slurry, there is an absence of work investigating any potential negative impact that this may
have on N and carbon (C) losses and on greenhouse gas (GHG) emissions. Brennan et al.
(2012) found in a plot study that chemical amendment of dairy cattle slurry with PAC
reduced ammonium-N (NH4+-N) runoff losses, but alum and lime led to increased NH4+-N
losses. All amendments reduced P losses in runoff, but had no effect on nitrate (NO3--N)
runoff losses. The Intergovernmental Panel on Climate Change (IPCC) (2007) estimates that
4 agricultural activities, including land application of animal manures, account for about 20%
of the anthropogenic global warming budget, with emissions principally comprised of
methane (CH4) from enteric fermentation and manure management and nitrous oxide (N2O)
from N application to soils. The EU 2020 Climate and Energy Package and its associated
Effort-Sharing Decision (Decision No 406/2009/EC) envisages reducing GHG emissions by
20% by 2020 across the whole of the EU. Whilst previous work has investigated the impact
of chemical amendments to pig slurry to reduce P in runoff (O’ Flynn et al., 2012a, 2012b),
no study has investigated the impact of chemical amendment of pig slurry on GHG
emissions.
Therefore, the aims of this laboratory study were to investigate if chemical amendment of pig
slurry: (1) reduced leached losses of N, P and C from a low P index soil (2) resulted in
changes to soil properties at different time intervals during the study period and (3) led to a
reduction in GHG emissions over 28 d from the time of application.
2. Materials and Methods
2.1 Slurry collection and characterisation
Pig slurry was taken from an integrated pig unit in Teagasc Research Centre, Moorepark,
Fermoy, Co. Cork, Rep. of Ireland in September 2011. The sampling point was a valve on an
outflow pipe between two holding tanks, which were sequentially placed after a holding tank
under the slats on which no bedding materials were used. To ensure a representative sample,
this valve was turned on and left to run for a few minutes before taking a sample. The slurry
was stored in a 25-L drum inside a cold-room fridge at 10oC prior to testing. The TP and total
5 nitrogen (TN) were determined using persulfate digestion. Ammonium-N was determined by
adding 50 ml of slurry to 1L of 0.1M HCl, shaking for 30 min at 200 rpm, filtering through
No. 2 Whatman filter paper, and analysing using a nutrient analyser (Konelab 20, Thermo
Clinical Labsystems, Finland). Total carbon was measured using a nutrient analyser
(Biotector, BioTector Analytical Systems Ltd, Ireland). Slurry pH was determined using a pH
probe (WTW, Germany). Dry matter (DM) content was determined by drying at 105oC for 24
h. The physical and chemical characteristics of the pig slurry used in this experiment and
characteristic values of pig slurry from other farms in Ireland are presented in Table 1.
2.2 Pig slurry amendment
Amendments for the present study were chosen based on effectiveness of P sequestration and
feasibility criterion (cost and potential environmental impediments) determined by O’ Flynn
et al. (2012a, 2012b). The amendment rates, which were applied on a stoichiometric basis,
were: (1) commercial grade liquid alum (8% Al2O3) applied at a rate of 0.88:1 [Al:TP] (2)
commercial-grade liquid ferric chloride (38% FeCl3) applied at a rate of 0.89:1 [Fe:TP], and
(3) commercial-grade liquid PAC (10 % Al2O3) applied at a rate of 0.72:1 [Al:TP].
Amendments were added to slurry in a 100-ml plastic cup and mixed for 10 s. The
compositions of the amendments used are shown in Table 2.
2.3 Soil collection and analysis
A sample of the plough layer (top 0.2 m) of an acid brown earth soil was collected from a
tillage farm in Fermoy, Co. Cork, Republic of Ireland. The site is typical of a free draining
soil, underlain by a karstified limestone aquifer. Tillage soil was chosen, as this type of soil is
6 often of a lower P index and is more suitable for the landspreading of pig manure. The soil
was air-dried, sieved (<2 mm) and thoroughly mixed. Soil samples (n=3) were oven dried at
40 °C for 72 h, crushed to pass a 2 mm sieve and analysed for Morgan’s P (Pm, the national
test used for the determination of plant available P in Ireland) using Morgan’s extracting
solution (Morgan, 1941). Soil total carbon (TC) and TN were determined by high
temperature combustion using a LECO Truspec CN analyser (LECO Corporation, St. Joseph,
MI, USA). Soil pH (n=3) was determined using a pH probe (WTW, Germany) and a 2:1 ratio
of deionised water-to-soil. The STP of the sample used in the column and batch experiments
was 3.21±0.29 mg L-1 (making it a P index 2 soil according to S.I. No. 610 of 2010), total
potassium (TK) of 41.8±3.00 mg L-1, TC of 1.84±0.05 %, TN of 0.19±0.00 %, C:N ratio of
9.87±0.22, a pH of 6.26±0.13, an organic matter (OM) content of 4.68±0.14%. A low range
STP tillage soil was chosen for this experiment to avoid the risk of background P from a high
range STP soil ‘masking’ the effect of each treatment. A low range STP tillage soil was also
chosen as present and future regulations will have the effect of making this type of land more
preferable for pig slurry spreading in the future.
The particle size distribution was determined using a sieving and pipette method (B.S.1377-2;
British Standards Institution (BSI), 1990a) and the organic content of the soil was determined
using the loss on ignition (LOI) test (B.S.1377-3; BSI, 1990b). The unstructured soil in the
column and batch experiments consisted of 57% sand, 29% silt and 14% clay, giving it a
sandy loam texture.
During any interaction with chemically amended slurry, the background soil P adsorption rate
must also be considered and can be assessed in a batch experiment following the procedure
outlined by Fenton et al. (2009). Ortho-phosphorus (PO43--P) solutions (90 ml), synthesised
7 using dissolved potassium phosphate (KH2PO4) in distilled water, ranging in concentration
from 4.1 to 28.9 mg P L-1, were added to 5 g samples of soil and shaken for 24 h using an
end-over-end shaker. Samples were passed through 0.45-µm syringe filters prior to being
analysed colorimetrically for dissolved reactive phosphorus (DRP) using a nutrient analyser
(Konelab 20, Thermo Clinical Labsystems, Finland). A Langmuir isotherm was used to
estimate the mass of P adsorbed per mass of the soil (McBride, 2000):
Ce
1 Ce


x
ab b
m
[1]
where Ce is the concentration of P in solution at equilibrium (mg L-1), x/m is the mass of P
adsorbed per unit dry weight of soil (g kg-1), a is a constant related to the binding strength of
molecules onto the soil, and b is the maximum adsorption capacity of the soil (g kg-1). In
conjunction with the P adsorption capacity of the soil, the equilibrium P concentration of the
soil (EPC0) (i.e. the point where no net desorption or sorption occurs) was derived using
(Olsen and Watanabe, 1957):
S′= kdC−S0
[2]
where S′ is the mass of P adsorbed from slurry (mg kg-1), C is the final P concentration of the
solution, kd is the slope of the relationship between S′ and C, and S0 is the amount of P
originally sorbed to the soil (mg L-1). The mass of P adsorbed per unit dry weight of soil was
0.224 g P kg-1 and the soil’s EPC0 was 0.513 mg L-1.
8 Soil water holding capacity (WHC) was determined according to Cassel and Nielsen (1986).
Soil was placed on a funnel whose sides were covered with Whatman no. 2 filter paper, and
distilled water was added to the soil until it became completely saturated. Saturated soil was
weighed, oven-dried overnight at 105oC, and weighed again.
Water-filled pore space, which can impact on rates of denitrification in soil, was estimated in
accordance with Haney and Haney (2010):
WFPS 
WC *  b
n
[3]
where ρb is bulk density and n is total porosity (mineral density was taken as 2.65 g cm-3).
Mineral N in soil (NH4+-N, NO3--N and nitrite-N (NO2--N)) was determined at 0, 7 and 28 d
after land application of pig slurry by adding 20 g of soil to 2 M KCl, shaking for 1 h,
filtering through No. 2 Whatman filter paper, and testing using a nutrient analyser (Konelab
20, Thermo Clinical Labsystems, Finland). Extra soil columns (n=3 for each treatment) were
set up to allow sampling after 7 d for soil mineral N.
2.4 Experimental columns
The experiment was conducted in 0.3-m-deep and 0.104-m-internal diameter columns with a
perforated stop-end inserted at the base to ensure that the soil remained free draining. A 0.05m layer of gravel, with a grain size of 5 – 10 mm, was placed at the base of each column.
Sieved soil (< 2 mm), previously mixed with distilled water to achieve a water content (WC)
of 26% (to replicate the average in situ field condition of the soil), was placed in 0.05 m-deep
increments in each column, so as the average dry bulk density was approximately 1.1 g cm-3
9 (equivalent to field conditions) and the total depth of soil was 0.2 m. At each depth
increment, soil was pressed along the wall of the column to avoid preferential flow
(Bhupinder Singh, pers. comm.).
The following treatments were examined: (1) soil only with no slurry applied (2) soil with
unamended slurry applied (the study control) and (3) soil receiving amended slurry. Slurry
was spread at 19 kg TP, 90 kg TN, and 180 kg TC ha-1. Columns were stored in a controlled
environment for 8 mo at 10o C at 75% humidity, based on typical climatic conditions in
Ireland (Walsh, 2012). All columns received 160 ml of distilled water wk-1, applied twice
weekly in two 80 ml increments over 2 h. This is equivalent to 980 mm of rainfall yr-1, or 19
mm wk-1, which would be in the mid-range of average annual rainfall amounts in Ireland
(Walsh, 2012). This application rate remained constant for the duration of the study;
however, actual rainfall rates will vary considerably over the course of a year. Drainage water
leachate was collected in plastic containers via funnels positioned under the perforated stopend of each column.
2.5 Leachate collection and analysis
The leachate from each column was composited and sampled weekly. Upon collection,
samples were weighed and a subsample was passed through a 0.45-µm filter and analysed
colorimetrically for DRP, NO2-, NH4+ and total oxidized nitrogen (TON) using a nutrient
analyser (Konelab 20, Thermo Clinical Labsystems, Finland). Nitrate was calculated by
subtracting NO2- from TON. Filtered and unfiltered subsamples were tested for total
dissolved phosphorus (TDP) and TP using acid persulfate digestion. Particulate phosphorus
(PP) was calculated by subtracting TDP from TP. Dissolved un-reactive phosphorus (DUP)
10 was calculated by subtracting DRP from TDP. Total nitrogen, total organic carbon (TOC)
and total inorganic carbon (TIC) were measured using a nutrient analyser (Biotector,
BioTector Analytical Systems Ltd, Ireland). Total carbon was calculated by adding TIC and
TOC. Leachate pH was determined using a pH probe (WTW, Germany). This addressed the
first aim of the study.
2.6 Destructive soil sampling
Soil columns were destructed after 1, 2, 3, 6 and 8 mo (n=3 for each treatment, at each time
period) and tested for WC, OM, pH, water extractable P (WEP), Pm, TN and TC. Before
analyses, each column was divided into 3 layers (0 to 0.05 m, 0.05 to 0.1 m, and 0.1 to 0.2 m
from the surface). Organic matter content of the soil was determined using the LOI test
(B.S.1377-3; BSI, 1990b). Soil pH was determined using a pH probe (WTW, Germany) and a
2:1 ratio of deionised water-to-soil. Water extractable P was measured by shaking 5 g of soil
in 25 ml of distilled water for 30 min, filtering through a 0.45-μm syringe filter, prior to being
analyzed colorimetrically for DRP (McDowell and Sharpley, 2001) using a nutrient analyser
(Konelab 20, Thermo Clinical Labsystems, Finland). Morgan’s P was determined using
Morgan’s extracting solution (Morgan, 1941). Soil TC and TN were determined for the
middle layer only in each column (0.05 to 0.1-m-depth) by high temperature combustion
using a LECO Truspec CN analyser (LECO Corporation, St. Joseph, MI, USA). This
addressed the second aim of the study.
2.7 Greenhouse gas emissions
11 Direct GHG emissions (N2O, carbon dioxide (CO2) and CH4) were analyzed over a 28-d
period in accordance with Troy et al. (2013). Samples were taken on the day of slurry
application (day 1) and subsequently on days 2, 3, 4, 5, 6, 7, 9, 11, 13, 15, 19, 23 and 28. The
top of the PVC columns were sealed using a rubber stopper. A sample of the air in the
headspace above the columns was taken through a rubber septum using a polypropylene
syringe with a hypodermic needle. The sample was immediately transferred into a preevacuated 7-ml screw cap septum vial. Samples were taken at 0, 5, 10 and 20 min after the
sealing of columns with a rubber stopper. After this period, the rubber stopper was removed.
Nitrous oxide, CO2 and CH4 concentrations were analysed using a gas chromatograph
(Varian CP 3800 GC, Varian, USA) fitted with a 63Ni electron capture detector (ECD) for
N2O analysis, a thermal conductivity detector (TCD) for CO2 analysis and a flame ionization
detector (FID) for CH4 analysis. During the analysis, 0.7 ml of a sub-sample from each vial
was drawn and injected first into a magnesium perchlorite (14-22 mesh) packed pre-column
to remove any moisture, followed by a 3-m-long, 3-mm-outside diameter stainless steel
column packed with Poropak Q (80/100 mesh). The column oven and injector temperature
were both 60°C and the detector temperature was 350°C. Argon (BOC Gases, Ireland),
flowing at 35 ml min-1, was used as a carrier gas. Samples were fed into the system by a
Combi-Pal automatic sampler (CTC Analysis, Switzerland) controlled by computer software.
Two-thirds of the injected sample was split to the ECD detector and one-third to the TCD and
FID in series. This allowed the simultaneous measurement of all three gases from the one
sample. Areas under the peaks were integrated using Star Chromatography Workstation
(Varian, USA). Fluxes were calculated from the change in headspace concentration over
measured period using:
V
 p  100  MW
1
dGas
 10 y 
 10 x  chamber
dt
R T
A
12 [4]
where: dGas is measured in ppm or ppb to get concentration at a certain point in time or ppm
h-1 or ppb h-1 to get the change in concentration over time; 10x is a recalculation (10-6 if
starting from ppm or 10-9 if starting from ppb); Vchamber is the volume of the chamber used; p
is atmospheric pressure; MW is the molecular weight either of N or N2O, depending of which
compound in which the emissions are expressed; R is a gas constant, 8314 J mol-1 K-1; T is
temperature in Kelvin; 10y is a recalculation (103 if the results are expressed in mg or 106 if in
µg); and A is the area of the chamber. The fluxes were then converted into mg m-2 d-1. Mean
daily emissions rates were calculated for each replicate by interpolation of values in between
the measurement days using arithmetic means (Velthof and Oenema, 1995; Flechard et al.,
2007). This addressed the third aim of the study.
2.8 Statistical analysis
The data was analysed in SPSS 20 (IBM, 2011) using a general linear model. Mean values of:
WC; OM; soil P, N and C species; soil pH; leachate P, N and C species; leachate pH; and
GHGs were analysed in a multivariate Tukey analysis when soil-only, slurry-only (the study
control), and slurry treated with alum, PAC and FeCl3 were applied. Data met the normal
distributional assumptions required. Probability values of p>0.05 were deemed not to be
significant.
3. Results
3.1. Water content, organic matter and soil pH
13 The WHC of the soil was found to equate to a WC of 53%. In general, there were no
significant differences observed in WC between treatments, apart from at 1 mo in the top soil
layer, where the soil-only treatment had a WC of 30.33±0.24% (data not shown).
Comparatively, at the same time, slurry-only, alum, FeCl3 and PAC treatments had WCs of
31.76±0.44%, 32.45±0.35%, 31.89±0.78%, and 32.13±0.39%. Water contents increased with
depth: WCs in the top soil layer were generally between 30 and 33%, between 31 and 34% in
the middle layer, and between 35 and 38% in the bottom layer. These equated to water-filled
pore space (WFPS) values of between 56 and 62% in the top layer, between 58 and 64% in
the middle layer, and between 65 and 72% in the bottom layer. Organic matter (generally
between 4.3 and 4.7%) and soil pH (between 6 and 6.5) were not significantly affected by
treatment, depth or time.
3.2. Nitrogen leachate and soil properties
There were no statistically significant differences between treatments for TN in soil (Table
3). No significant differences between treatments were observed for the N in leachate water,
which mainly comprised NO3-. The amount of NO3- leached increased rapidly until wk 2,
before it reduced gradually thereafter (Fig. 1 c). Approximately 95% of TN leached from the
columns over the duration of the studies was in the form of NO3-, with roughly 0.2% in the
form of NO2- and 0.3% in the form of NH4+. The C:N ratio for all treatments at all destructive
periods was between 9 and 10 (Table 3). Nitrite loads peaked between wks 10 and 26 (Fig. 1
b).
At all times, mineral N in soil comprised less than 2% of soil TN. Seven days after
application, soil NH4+ was observed to be highest for the alum and FeCl3 treatments (83.7
14 and 79.3 g NH4+-N kg-1 soil, respectively). This compared with values of 44.0 and 48.9 g
NH4+-N kg-1 soil for soil-only and slurry-only, respectively.
3.3. Nitrous oxide emissions
Nitrous oxide emissions from the soil-only treatment remained fairly constant throughout the
28-d study (Fig. 2 a), with cumulative emissions of 22±8 mg N2O-N m-2. Application of pig
slurry led to an increased cumulative release of N2O. Cumulative emissions across all Napplied treatments were high, ranging approximately from 60 to 200 mg N2O-N m-2. The
highest cumulative losses of 188±86 mg N2O-N m-2 was observed for FeCl3-amended slurry
and this was the only treatment statistically significantly different (p=0.008) to soil-only, but
was not statistically significantly different to any other treatment. Cumulative emissions from
all treatments remained relatively constant between 4 and 7 d after application of slurry, at
which point they increased more rapidly, although not significantly, and continued to rise
until the end of the study. However, N2O losses from FeCl3–amended slurry were at all times
greater than all other treatments. Alum and PAC-amended slurries both had less, but not
statistically significantly different, N2O losses than unamended slurry, but more than soilonly.
3.4. Phosphorus leachate and soil properties
There were no significant differences in the quantity of P leached between treatments, with
the majority of TP made up of TDP for all treatments. Particulate phosphorus comprised
approximately 30% of the TP load in all cases.
15 In general, there were no significant differences in levels of Pm and WEP between treatments
in the bottom two soil layers (Table 3). However, in the top soil layer, application of
unamended slurry resulted in increased Pm and WEP, which were significantly different
(p<0.05) to the soil-only columns at all destructive periods (Table 3). Levels of Pm and WEP
in the top soil layer were both reduced by the application of amended slurry to levels not
significantly different to soil-only columns (Table 3).
3.5. Carbon leachate
The average cumulative amount of TOC and TIC leached is shown in Fig. 3 a. The average
TC leached from the soil-only columns was 217.3 mg. This increased to 253 mg from
columns with unamended slurry, with reduced amounts of TC leached from columns treated
with amended slurry. However, there were no statistically significant differences for TC loads
between treatments. There was an increase in loads of TC leached from wk 1 to wk 2 (Fig. 3
b); however, this was due to lower leachate volumes during wk 1 than wk 2, rather than any
changes in concentration. The loads of TC leached then decreased after wk 2 until the end of
the study, during which time there was no significant change in flows.
3.6. Carbon emissions
Emissions of CO2 followed a similar trend to N2O emissions (Fig. 2 b). The soil-only
treatment had the lowest emissions, with cumulative losses of 36±4 g CO2-C m-2. Losses
increased upon application of slurry, but were only statistically significantly different
(p=0.008) in the case of FeCl3-amended slurry, which had cumulative losses of 106±23 g
CO2-C m-2. However, this was not statistically significantly different to any other unamended
16 or amended slurry treatment. Alum and PAC-amended slurries had less, but not statistically
significant different, losses than unamended slurry. Methane losses were highly variable (Fig.
2 c), but no treatment had significantly higher losses than the soil-only treatment. After 5
days, all treatments either gained or lost CH4, with FeCl3–amended slurry acting overall as a
net sink with cumulative losses of -13±7 mg CH4-C m-2, whilst PAC-amended slurry had
cumulative losses of 13±6 mg CH4-C m-2.
4. Discussion
4.1 Nitrogen leachate and soil properties
Denitrification is the mainly microbial reduction of NO3--N to the gaseous products nitric
oxide (NO), N2O, or inert di-nitrogen (N2). Some studies have shown that the highest rates of
denitrification occur in the upper soil horizon (Kustermann et al., 2010; Jahangir et al., 2012),
the extent of which depends on WC and WFPS. Soil WC can impact on many different soil
processes such as mineralization, leaching, plant uptake and denitrification (Porporato et al.,
2003).
The early peak in NO3- loss may be due to the drying and re-wetting during column
construction, which could have caused a surge in microbial activity and C and N
mineralisation (Van Gestel et al., 1991; Bengtsson et al., 2003). This may also have led to an
early peak in leachate NH4+ (Fig. 1 a). Once rewetting was complete, WFPS levels were
between 65 and 72% in the bottom layer. At WFPS levels of over 60%, denitrification may
take place, releasing nitrogen gas (N2) and N2O into the atmosphere (Porporato et al., 2003).
Aerobic microbial activity and nitrification is also reduced in these anaerobic conditions
17 where denitrification is facilitated (Poporato et al., 2003; Rivett et al., 2008). The fractions of
NO2-, NO3- and NH4+ in the leachate would seem to indicate that almost complete
nitrification occurred, and also led to the drop in NO3- levels after wk 2. This hypothesis was
also supported by the C:N ratios present (Table 3). Soil with C:N ratios below 20 can be
characterised as having a surplus of available NH4+ for nitrification (Bengtsson et al., 2003).
The peak in NO2- between wks 10 and 26 may have been due to a delay in reduction of NO2during denitrification down to the preference of denitrifiers for NO3-, even when both are
present (Rivett et al., 2008).
High NH3+ volatilization may occur after land application of pig slurry, with over 60% of
total losses occurring in the first 10 h after application (Gordon et al., 2001; Rochette et al.,
2001). It would appear in the current study that a large amount of volatilization occurred from
both amended and unamended slurry treatments with little unvolatilised inorganic N
remaining, which is in agreement with previous studies (Morvan et al., 1997; Hoekstra et al.,
2010; Hoekstra et al., 2011). Indeed, these rates of volatilization may represent a loss of 5080% of total ammoniacal nitrogen from landspread slurry over a 10-d period (Misslebrook et
al., 2005a, 2005b; Meade et al., 2011). The slurry organic fraction was undetectable in
leachate or soil (Table 3) due to the large background amounts of soil inorganic N, which was
a result of the occurrence of mineralization. Unlike the present study, which found no
significant difference between NO3- losses from columns with and without slurry spread on
them, Daudén et al. (2004) found that drainage NO3- concentrations and loads consistently
increased with increasing amount of N applied when landspreading pig slurry and mineral
fertiliser between 275 and 1487.5 kg N ha-1. However, the spreading rate used by Daudén et
al. (2004) was much higher than in the present study (90 kg N ha-1), and in that study, pig
slurry was incorporated into soil to minimise volatilization losses.
18 4.2 Nitrous oxide emissions
The increased cumulative release of N2O after slurry application was as expected (Velthof et
al., 2003). The cumulative N2O emissions across all N-applied treatments represented a loss
of between 1% and 3% of applied total ammoniacal N for a 28-d period. This was a higher
emission factor than the IPCC default emission factor of 1% (IPCC, 2006). Generally, higher
emission factors would not be associated with free-draining soil such as the one used in this
study (Abdalla et al., 2009; Rafique et al., 2011). However, emission factors associated with
slurry application have previously been observed to be higher than the default values and this
may be related to the simultaneous application of a labile C source, which increases microbial
activity (Dendooven et al., 1998; Sherlock et al., 2002). Nitrous oxide is produced by both
nitrification and denitrification (Chadwick et al., 2011), and can be influenced by oxygen
availability, soil WC, soil temperature, soil NO3- and organic carbon content (Section 4.4)
(Velthof et al., 2003). The drying and rewetting of the soil during construction provided
conditions which facilitated C and N mineralisation and denitrification, would also have
facilitated N2O release to the atmosphere (Porporato et al., 2003).
The increase in N2O emissions associated with FeCl3 addition may be explained as a result of
ammonia volatilisation abatement. The difference in soil NH4+ levels between treatments 7 d
after application was due to a reduction in volatilisation, possibly resulting from a reduction
in slurry pH upon amendment addition. Previous work has observed that volatilisation may
be reduced upon FeCl3 addition, principally due to a reduction in slurry pH (Molloy and
Tunney, 1983).
19 4.3 Phosphorus leachate and soil properties
Unlike previous runoff studies (O’ Flynn et al., 2012b), in which spreading of pig slurry led
to a large increase in all types of P in runoff compared to runoff from soil-only, there were no
significant differences in the quantity of P leached between treatments. The fraction of TP
load made up of TDP was less when compared to O’ Flynn et al. (2012b), who found PP in
runoff comprised, on average, 45% of TP. This is in agreement with McDowell et al. (2004),
who found that more TP was lost as PP in overland than subsurface flow due to the higher
kinetic energy and erosive power of high-frequency storms. Loss of P in subsurface flow is
generally less than that in runoff, and will decrease as the degree of soil–water contact
increases, due to sorption by P-deficient subsoils (Haygarth et al., 1998; McDowell et al.,
2004). Although a soil with a low Pm (3.21±0.29 mg L-1) was used in this experiment, its
high adsorption capacity for P (0.224 g P kg-1) and low EPC0 (0.513 mg L-1) facilitated
adsorption of P during leaching.
The same amendments and application rates as used in the present study were also used by O’
Flynn et al. (2012a), who achieved reductions of between 95 and 99% in the WEP of slurry.
Dao (1999) amended stockpiled cattle manure with caliche, alum and flyash in an incubation
experiment, and reported WEP reductions in amended manure, compared to the study
control, of 21, 60 and 85%, respectively. Similarly, in a study that examined the effect of soil
P level in a silt loam soil which was incubated at 25°C, Kalbasi and Karthikeyan (2004)
reported that applications of alum and FeCl3-amended slurry to soil decreased soil WEP. In
the present study, due to the regular application of 160 ml water wk-1, which led to the
downward leaching of P from the slurry, both Pm and WEP levels in the columns spread with
unamended slurry reduced to levels closer, but still significantly different (p<0.05), to soil-
20 only and amended slurry columns. This P was adsorbed by the soil’s high adsorption capacity
for P, but was not detected by WEP or Pm analysis.
4.4 Carbon leachate and emissions
The decrease in loads of TC leached after wk 2 may have been due to the increased
mineralization of C and N, which may have been the cause of increased losses of CO2 to the
atmosphere. This loss of CO2 to the atmosphere may also be the reason that there were
statistically no significant differences between treatments for TC in soil (Table 3). In
addition, organic carbon can act as an electron donor to facilitate the occurrence of
denitrification when anaerobic conditions are present (Rivett et al., 2008).
The addition of manure slurries to soil has been shown to cause an increase in microbial
activity and CO2 emissions (Bol et al., 2004; Dumale et al., 2009; Cayuela et al., 2010). The
increased CO2 losses from unamended or amended slurry treatments were in agreement with
the hypothesis that these losses were the cause for no statistically significant differences
between slurry treatments for TC in soil (Table 3).
After land application, CH4 emissions are generally of minor importance compared to N2O
emissions (Wulf et al., 2002a, 2002b), as CH4 emissions from enteric fermentation and
during slurry storage are much more important (Chadwick et al., 2000). This is due to CH4
being produced by decomposition of OM in faecal matter under anaerobic conditions. After
landspreading, OM is oxidised to CO2 and H2O in the aerobic conditions present. Mineral
grassland soils are known to generally be a CH4 sink, due to either oxidation of CH4 to CO2
in soils or incorporation into microbial biomass, with uptake rates ranging from 0.5 – 3.3 mg
21 CH4 m-2 d-1 (Mosier et al., 1991; Dobbie et al., 1996; Saggar et al., 2008). The results from
the present study show that no additional risk to CH4 emissions is posed by the chemical
amendment of pig slurry.
4.5 Outlook for use of chemical amendment as a mitigation measure
Increased intensification of pig farming activities, along with legislation reducing the amount
of land onto which pig farmers may apply slurry, has meant that the pig industry is under
increasing pressure to reconcile production and water quality objectives. Land application of
pig slurry is currently the most cost-efficient method for its disposal. In Ireland, the pig
industry is concentrated in a small number of areas, with typically high stocking rates.
Therefore, the disposal of slurry in a cost-effective and environmentally responsible way is a
serious issue for farmers.
This study demonstrates that amendments previously selected on the basis of ability to reduce
runoff P (O’ Flynn et al., 2012a,b), may be used without posing a negative impact on
leachate, soil properties, and GHG emissions.
Based on the results of the current study and also previous work by the authors comparing
cost (O’ Flynn et al., 2012a) and surface runoff losses (O’ Flynn et al., 2012b), PAC appears
to be the most suitable amendment with which to chemically amend pig slurry. Ferric
chloride resulted in increased N2O and CO2 losses, whereas alum and PAC resulted in
reduced, but not significantly different, losses to slurry-only. Poly-aluminium chloride
performed best in overall removal of runoff P and SS (O’ Flynn et al., 2012b). There was
little difference between leachate losses and soil effects from alum and PAC-amended slurry,
22 although this study only included one soil type. The current study used a low STP soil so as
to avoid the risk of background P from a high range STP soil ‘masking’ the effect of each
treatment. However, future work must examine a wide variety of soil types, including high
STP soils. These amendments must also be examined at field-scale, and include repeated
application and incorporation. Costs were comparable (O’ Flynn et al., 2012a), with
estimated costs of amending and spreading amended slurry of €3.33 and €3.69 m-3 for alum
and PAC, respectively, in comparison to €1.56 m-3 to spread unamended slurry.
In the current study, reductions were not adequate to satisfy the EU 2020 Climate and Energy
Package of reducing GHG emissions by 20% across the whole of the EU by 2020. It has
however, been shown that some reductions are possible, and future work must be carried out
to identify if more significant reductions in GHG emissions is possible at different
application rates.
At present, there is no provision in legislation for chemical amendments to be used as a
mitigation measure in the land application of pig slurry, but if they are to be utilised, a
regulatory framework will need to be introduced by the relevant bodies.
5. Conclusions
Chemical amendment of land applied pig slurry can reduce P in runoff without any negative
impact on nutrient leaching. Furthermore, there were no significant differences between
treatments for N and C in leachate or soil, indicating no deleterious impact on reactive N
emissions or soil C cycling. Chemical amendment posed no significant change to GHG
emissions from pig slurry, and in the cases of alum and PAC, reduced cumulative N2O and
23 CO2 losses. Moreover, increased N2O emissions associated with FeCl3 addition were likely to
be due to a reduction in ammonia volatilisation, a theory supported by an increase in soil
NH4+ concentrations.
Acknowledgements
The first author gratefully acknowledges the award of the EMBARK scholarship from the
Irish Research Council to support this study. The authors would like to thank Dr. Raymond
Brennan, Dr. John Regan and Swann Lamarche.
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35 Figure Captions
Figure 1. Average weekly loads of ammonium a) nitrite b) and nitrate c) leached column-1 (±
standard deviation).
Figure 2. Cumulative gaseous emissions of N2O-N (a) CO2-C (b) and CH4-C (c) from
columns at each sampling period (± standard deviation).
Figure 3. Cumulative loads of total organic carbon (TOC) and total inorganic carbon (TIC)
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36 Table 1. Physical and chemical characteristics of the pig slurry used in this experiment and
characteristic values of pig slurry from other farms in Ireland.
TP
TN
TC
NH4+-N
pH
(mg L-1)
620±32
2940±156 5860±80
DM
Reference
(%)
1739 ±8
7.51 ± 0.08
3.02± 0.24
The present study
800
4200
S.I. No. 610 of 2010
1630
6621
5.77
McCutcheon, 1997a
900±7
4600±21
3.2±2.3
O’ Bric, 1991a
TP, total P; TN, total N; TK, total K; DM, dry matter. aValues changed to mg L-1 assuming densities of 1 kg
L-1.
37 Table 2. Characterisation of amendments used in this study (O’ Flynn et al., 2012a, 2012b)
Amendment
pH
Alum
Ferric
Chloride
PAC
8% Al2O3
38% FeCl3
10 % Al2O3
1.25
WEP
mg kg-1
0
Al
%
4.23
1.0 – 3.0
Fe
%
<0.01
38
As
mg kg-1
1
<2.8
<1.0
Cd
-1
mg kg
0.21
<3.4
<0.2
Cr
mg kg-1
2.1
<48
<2.0
-1
Cu
mg kg
<65
Mn
mg kg-1
<1370
Ni
mg kg-1
1.4
<48
<1.0
Pb
mg kg-1
2.8
<14
<2.0
Sb
-1
<2.8
<1.0
Se
-1
mg kg
<2.8
<1.0
Hg
mg kg-1
<0.7
<0.2
mg kg
38 Table 3. Average soil phosphorus, nitrogen and carbon contents by sampling time and depth.
Treatment
Month Depth (mm)
Soil Only Slurry
Alum
PAC
FeCl3
Morgan's P
7.79 c
4.19 ab
4.64 b
4.40 ab
1
0-50
3.53 a
a
a
a
a
-1
3.69
3.80
3.75
3.69
3.68 a
(mg L ) 50-100
a
a
a
a
3.99
3.79
3.95
3.84 a
3.53
100-200
6.12 b
4.41 a
4.61 a
4.52 a
3.84 a
2
0-50
a
a
a
a
4.02
4.03
3.85
3.80
3.99 a
50-100
a
a
a
a
4.31
3.88
3.86
4.08 a
4.14
100-200
6.28 c
4.22 b
4.55 b
4.28 b
3.19 a
3
0-50
a
a
a
a
3.14
3.17
3.50
3.60
3.39 a
50-100
a
a
a
a
3.55
3.71
3.78
3.67 a
3.35
100-200
4.60 c
3.44 ab
4.18 bc
3.52 ab
2.69 a
6
0-50
a
a
a
a
3.22
3.41
3.21
3.62
3.10 a
50-100
a
a
a
a
3.67
3.65
3.61
3.28 a
3.51
100-200
3.42 c
2.63 ab
3.00 bc
3.38 c
2.17 a
8
0-50
a
ab
ab
ab
2.44
2.39
2.67
2.95
3.16 b
50-100
a
a
a
a
2.66
3.14
3.01
3.38
3.66 a
100-200
WEP
1.13 b
0.49 a
0.57 a
0.59 a
1
0-50
0.54 a
a
a
a
a
-1
0.56
0.58
0.54
0.58
0.57 a
(mg kg ) 50-100
0.56 a
0.57 a
0.60 a
0.54 a
0.64 a
100-200
a
b
a
a
0.51
0.99
0.57
0.57
0.55 a
2
0-50
a
a
a
a
0.49
0.47
0.49
0.45
0.50 a
50-100
0.46 ab
0.39 b
0.43 ab
0.45 ab
0.50 a
100-200
a
b
a
a
1.06
0.69
0.71
0.73 a
0.62
3
0-50
a
a
a
a
0.65
0.66
0.61
0.67
0.62 a
50-100
0.70 a
0.65 a
0.63 a
0.62 a
0.64 a
100-200
a
b
a
a
0.87
0.60
0.63
0.52 a
0.54
6
0-50
a
a
a
a
0.54
0.55
0.50
0.52
0.49 a
50-100
0.51 a
0.47 a
0.47 a
0.44 a
0.49 a
100-200
a
b
a
a
0.79
0.55
0.56
0.62 ab
0.58
8
0-50
a
a
a
a
0.58
0.62
0.55
0.53
0.57 a
50-100
0.61 a
0.58 a
0.57 a
0.56 a
0.55 a
100-200
a
a
a
a
TC
1.73
1.86
1.69
1.74 a
1
50-100
1.70
a
a
a
a
(%) 2
50-100
1.78
1.73
1.77
1.76
1.68 a
3
50-100
1.72 a
1.73 a
1.74 a
1.84 a
1.68 a
a
a
a
a
6
50-100
1.81
1.78
1.74
1.79
1.66 a
a
a
a
a
1.73
1.73
1.79
1.75 a
8
50-100
1.75
TN
0.18 a
0.20 a
0.19 a
0.19 a
1
50-100
0.18 a
a
a
a
a
(%) 2
50-100
0.18
0.18
0.18
0.19
0.18 a
a
a
a
a
3
50-100
0.18
0.18
0.18
0.19
0.18 a
6
50-100
0.19 a
0.19 a
0.18 a
0.18 a
0.18 a
a
a
a
a
0.19
0.18
0.18
0.18 a
8
50-100
0.19
C:N Ratio
9.39 a
9.32 a
9.07 a
9.30 a
1
50-100
9.53 a
a
a
a
a
2
50-100
9.73
9.89
9.69
9.41
9.30 a
a
a
a
a
3
50-100
9.54
9.61
9.51
9.80
9.39 a
a
a
a
a
6
50-100
9.38
9.43
9.78
9.78
9.32 a
9.35 a
9.79 a
10.04 a
9.76 a
8
50-100
9.31 a
abc
Means in a row, which do not share a superscript, were significantly different (p< 0.05)
39 Fig 1.
a)
0.10
Ammonium(mg)
0.08
0.06
0.04
0.02
0.00
0
5
10
15
20
25
30
35
Week
b)
0.05
Nitrite (mg)
0.04
0.03
0.02
0.01
0.00
0
5
10
15
20
25
30
35
20
25
30
35
Week
c)
25
Nitrate (mg )
20
15
10
5
0
0
5
10
15
Week
40 Fig 2.
mg N2O-N m-2
a)
1 2 3 4 5 6 7
9
11
13
15
Day
19
23
28
1 2 3 4 5 6 7
9
11
13
15
Day
19
23
28
1 2 3 4 5 6 7
9
11 13
15
Day
g CO2-C m-2
b)
mg CH4-C m-2
c)
41 19
23
28
Leachate Carbon (mg)
Fig 3.
a)
b)
12
Leachate Carbon (mg)
10
8
6
4
2
0
0
5
10
15
Week
42 20
25
30
35
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