mitigation of greenhouse gas emissions by anaerobic digestion of cattle slurry

Mitigation of greenhouse gas emissions by anaerobic

digestion of cattle slurry

Joachim Clemens a,*, Manfred Trimborn a, Peter Weiland b, Barbara Amon c

aUniversity of Bonn, Institute of Plant Nutrition, Karlrobert-Kreiten-Strasse 13, D-53115 Bonn, Germanyb Institute of Technology and Biosystems Engineering, Federal Agricultural Research Centre (FAL),

Bundesallee 50, D-38116 Braunschweig, GermanycUniversity of Natural Resources and Applied Life Sciences, Department of Sustainable Agricultural Systems,

Division of Agricultural Engineering, Peter Jordan-Strasse 82, A-1190 Vienna, Austria

Available online 7 November 2005

Abstract

Biogas treatment of animal manures is an upcoming technology because it is a way of producing renewable energy (biogas). However,

little is known about effects of this management strategy on greenhouse gas (GHG) emissions during fermentation, storage, and field

application of the substrates compared to untreated slurries. In this study, we compared cattle slurry and cattle slurry with potato starch as

additive during the process of fermentation, during storage and after field application. The addition of potato starch strongly enhanced CH4

production from 4230 l CH4 m�3 to 8625 l CH4 m

�3 in the fermenter at a hydraulic retention time (HRT) of 29 days. Extending the HRT to 56

days had only a small effect on the CH4 production. Methane emissions from stored slurry depended on storage temperature and were highest

from unfermented slurry followed by the slurry/starch mixture. Gas emissions from untreated and fermented slurry during storage were

further analyzed in a pilot-scale experiment with different levels of covering such as straw cover, a wooden lid and no cover. Emissions of

greenhouse gases (CH4, N2O, NH3) were in the range of 14.3–17.1 kg CO2 eq. m�3 during winter (100 day storage period) and 40.5–90.5 kg

CO2 eq. m�3 during summer (140 day storage period). A straw cover reduced NH3 losses, but not overall GHG emissions, whereas a solid

cover reduced CH4 and NH3 emissions. After field application, there were no significant differences between slurry types in GHG emissions

(4.15–8.12 kg CO2 eq. m�3 a�1). GHG emissions from slurry stores were more important than emissions after field application. Co-digestion

of slurry with additives such as starch has a large potential to substitute fossil energy by biogas. On a biogas plant, slurry stores should be

covered gas-tight in order to eliminate GHG emissions and collect CH4 for electricity production.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Biogas; Greenhouse gases; Nitrous oxide; Methane; Ammonia; Slurry; Manure management

www.elsevier.com/locate/agee

Agriculture, Ecosystems and Environment 112 (2006) 171–177

1. Introduction

In biogas plants, organic matter is degraded by anaerobic

digestion to ‘‘biogas’’, a mixture of methane (CH4), carbon

dioxide (CO2), and some trace gases. Biogas is a source of

renewable energy producing electricity in combined heat

and power plants. In agriculture, the digestion of slurry or

liquid manure is the major source of biogas. The co-

digestion of slurry with organic wastes, residues and/or

* Corresponding author. Tel.: +49 228 732150; fax: +49 228 732489.

E-mail address: [email protected] (J. Clemens).

0167-8809/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2005.08.016

energy crops is of increasing relevance because it increases

CH4 production and improves the profitability of the process

(Weiland et al., 2004).

The biogas yield depends on the temperature in the

fermenter, on the HRT, and on the specific organic load.

During fermentation, substrate parameters such as organic

dry matter content, ammonia concentration, pH, and

viscosity undergo changes that may affect GHG emissions

during storage and after field application of residues. For

example an increase in pH and NH4+ concentration may

stimulate NH3 emission during storage and after field

application (Kuhn, 1998). On the other hand, CH4 emission

J. Clemens et al. / Agriculture, Ecosystems and Environment 112 (2006) 171–177172

may be reduced because most of the degradable organic

carbon is turned into biogas (Huther, 1999). Studies on N2O

emission during storage were inconsistent, while Sommer

et al. (2000) reported periods with higher N2O emission,

other authors found only negligible N2O emission (Wulf

et al., 2002b). Some studies have observed that the presence

of a surface crust reduced CH4 emissions (Husted, 1994;

Sommer et al., 2000), indicating that CH4 was consumed

within the crust environment by CH4 oxidising bacteria. In

Dutch studies, a solid cover on slurry stores reduced CH4

emissions by up to 50% (Hilhorst et al., 2001). Pilot scale

experiments were conducted to show the effect of different

levels of covering slurry stores on CH4, NH3, and N2O

emissions.

After field application of slurry, GHG emissions are

dominated by N2O and NH3, whereas CH4 is of minor

importance (Wulf et al., 2002b).

In this study, we investigated the influence of all three

major steps of anaerobic digestion, fermentation, storage,

and field application, on GHG and NH3 emissions, and we

compared these results with emissions from untreated

slurries. Other effects of NH3 emissions, such as eutrophica-

tion and acidification (Ellenberg, 1992), are not considered

in this study. Our hypothesis was that biogas production

reduces overall GHG emissions during slurry management.

2. Material and methods

2.1. Fermentation studies

Digestion experiments were carried out with cattle slurry

and a mixture of cattle slurry and potato starch (Table 1) in a

farm-scale digester (600 m3) and a pilot-scale digestor

(8 m3) at the experimental station of the Federal Agricultural

Research Centre (FAL) in Braunschweig/Germany. Usually,

agricultural biogas plants are operated under mesophilic

conditions, and the HRT is 20–35 days (Weiland et al.,

2004). Here, the digesters were operated at 35 8C with a

HRTof 29 or 56 days. The substrate was fed to the digesters

Table 1

Properties of cattle slurry and a mixture of cattle slurry and waste potato starch (OD

retention time (HRT) of 29 and 56 days

Parameter HRT Cattle

slurry

Cattle slurry/

potato starch

Cattle slurry,

29 days

DM (g kg�1) 32.9 43.7 22.9

ODM (g kg�1) 23.5 34.5 14.5

COD (g kg�1) 39.8 58.9 21.7

BOD (g kg�1) 16.5 20.4 4.3

NKj (g kg�1) 2.17 2.72 2.06

NH4–N (g kg�1) 1.19 1.45 1.41

PO4–P (g kg�1) 0.40 0.46 0.28

FOS/TAC 0.83 1.25 0.1

pH 7.4 7.6 7.6

DM: dry matter; ODM: organic dry matter; COD: chemical oxygen demand; BOD

acids/total acid capacity.

12 times per day, while the digested substrate was removed

continuously. Production and CH4 content of the biogas

were measured continuously. All digesters were operated at

constant operational conditions for at least one HRT before

the substrate was collected for field application experiments.

2.2. Storage studies

Two storage experiments were performed. Small scale

experiments were conducted at the FAL in order to quantify

the potential for CH4 formation during storage. For this

purpose, 25 kg of fresh, digested slurry and digested slurry/

starch mixtures were stored in gas-tight glass vessels at four

different temperatures: 4, 11, 20, and 30 8C. The gas

produced was collected in gas-tight bags. Depending on the

gas productivity, the gas volume was measured with

conventional wet gas meters or with special micro gas

meters. All laboratory storage experiments were carried out

in duplicate with only minor differences in the CH4

emission. The gas from fermentation and laboratory storage

studies was analyzed by gas chromatography (Shimadzu GC

14B) using a flame ionization detector (FID) for CH4, a

thermal conductivity cell (WLD) for CO2, and an electron

capture detector for N2O (for details, see Huther, 1999).

Additionally, a pilot-scale study was performed in Gross

Enzersdorf, near the city of Vienna, Austria, at the research

station of the University of Natural Resources and Applied

Life Sciences. Ammonia, CH4, and N2O emissions were

quantified during storage of untreated and digested dairy

cattle slurry in a summer (140 days) and winter (100 days)

experiment. The following treatments were included:

untreated slurry with natural surface crust (untr_crust),

untreated slurry with natural surface crust and wooden cover

(untr_cover), digested slurry without any cover (biogas),

digested slurry with a layer of chopped straw (biogas_straw),

digested slurry with a layer of chopped straw and a wooden

cover (biogas_straw_cover).

The dairy cattle slurry was stored in five pilot-scale slurry

tanks, which were 2.5 m deep and with a diameter of 2.5 m.

Emissions were quantified by placing a large, open dynamic

M-ratio: 70/30) and main properties of the digested substrates at a hydraulic

Cattle slurry,

56 days

Mixture 70/30,

29 days

Mixture 70/30,

56 days

23.1 27.6 22.8

14.3 17.5 14.3

19.7 27.7 33.7

3.4 16.4 8.0

2.28 2.73 2.46

1.51 1.78 1.65

0.37 0.25 0.26

0.2 0.1 0.1

7.8 7.6 7.8

: biological oxygen demand, NKj: Kjeldahl-N, VOA/TAC: volatile organic

J. Clemens et al. / Agriculture, Ecosystems and Environment 112 (2006) 171–177 173

Table 2

Methane formation (l m�3 slurry) during anaerobic digestion at 35 8C with

an average HRT of 29 or 56 days

Slurry Slurry + starch

ODM (g kg�1) 23.5 34.5

HRT (d) 29 56 29 56

Methane yield (l m�3) 4230 4465 8625 10350

ODM: organic dry matter.

chamber over a slurry tank and monitoring the emissions for

several hours at least twice a week. For details on the

measurement technology, see Amon et al. (this issue).

Regression curves were fitted to cumulated emissions.

Cumulated emissions of CH4, NH3, and N2O increased

linearly during the winter trial, and this was also the case for

N2O in the summer trial. Methane and NH3 emissions in the

summer trial showed a quadratic increase, i.e. emission rates

decreased over time (Amon et al., 2004).Regression equations

were testedbypair-wise comparisons of regressionparameters

using the t-test. Level of significance was set to at least 0.05.

2.3. Field application

In the field experiment, the substrates produced by the

FAL, except untreated cattle slurry/potato starch and

digested slurry with a HRT of 56 days (Table 1), were

applied by manual simulation of the trail shoe technique.

Rates of 60 kg NH4–N ha�1 were applied to a pasture on a

poorly drained Stagno-gleyic Luvisol (FAO). C and N

contents of the grassland topsoil (0–0.15 m) were 1.9 and

0.19%, respectively. The plot experiment (four replications

for each treatment, plot size: 9 m2) also included an

unfertilised control and a mineral fertiliser treatment

(Calcium Ammonium Nitrate, CAN). Application of the

substrate on each single plot took about 5 min, and gas flux

measurement started immediately thereafter. The substrates

were applied to all plots within 90 min.

Nitrous oxide and CH4 emissions were measured for 1

year after application using closed chambers with a surface

area of 0.25 m2 and a volume of 96 l. Sampling frequency

was reduced from four measurements per day during the first

week to one measurement per month at the end of the 1-year

period. Gas samples were taken from the chambers with

evacuated headspace vials (20 ml) through a butyl septum.

During the first week, samples were taken 0, 30, 60, and

90 min after placing the chambers airtight onto installation

rings that were permanently inserted 10 cm into the soil. In

the following weeks, the sampling intervals were 0, 45, 90,

and 135 min. Gas analysis was performed using a gas

chromatograph (SRI 8610C) with a backflush system to

eliminate water vapour, an electron capture detector (ECD)

for N2O and a flame ionisation detector (FID) for CH4

measurements (for details, seeWulf et al., 2002b). Ammonia

volatilisation was measured during the first 4 days after

application using the standard comparison method

described by Vandre and Kaupenjohann (1998) andmodified

by Wulf et al. (2002a). This is an open method designed for

plot experiments where NH3 is collected in passive samplers

filled with 0.05 M H2SO4.

Results are presented as arithmetic means of four

replicates. Although emission measurements are often not

normally distributed, this estimator was preferred to

geometric or lognormal means because according to Velthof

and Oenema (1995), it is less biased and more robust than

any other estimator for small numbers of replicates. For

comparison of treatments, ANOVA was used followed by

Tukey-HSD test using SPSS version 10 (2000) software.

In order to compare total GHG emissions, emissions of

each gas were converted to CO2 equivalents using IPCC

conversion factors (IPCC, 1996). In this study, NH3 emissions

were taken into account in the calculation of the GHG

emissions. Ammonia is not itself considered a greenhouse gas

because of its short lifetime in the atmosphere, but its

deposition induces N2O formation elsewhere. It is postulated

that 1% of NH3–N deposited is re-emitted as N2O–N (IPCC/

OECD/IEA, 1997). Residual C in the slurry not released as

CH4 was not considered in the calculation.

3. Results

3.1. Fermentation studies

The addition of potato starch to slurry increased CH4

formation during the fermentation. Extending the HRT from

29 to 56 days had only a minor effect on CH4 yields from

cattle slurry (increase of 235 l CH4 m�3), whereas yields

from the slurry/starch mixture increased significantly by

1725 l CH4 m�3 substrate (Table 2) with the extended HRT.

3.2. Storage

Results from laboratory storage experiments indicated a

strong influence of the storage temperature on CH4

formation. From untreated slurry, CH4 emissions can be

neglected only at the lowest storage temperature of 4 8C.From digested substrates, the amount of CH4 emitted was

generally lower. The digested slurry/starch mixture pro-

duced more CH4 as compared to digested slurry. Mean CH4

emissions from untreated and anaerobically digested slurry

with and without starch are summarized in Table 3,

expressed as CO2 equivalents. No N2O emissions were

detected in the experiments.

The pilot-scale storage experiment included winter and

summer storage. Table 4 gives an overview of net total

emissions measured under cool winter and warm summer

conditions.

A linear increase in cumulated CH4 emissions was

observed in all treatments throughout the winter experiment

(Amon et al., 2004). Methane emissions from digested slurry

were significantly lower than from untreated slurry

(Table 4). No significant difference was observed between


Table 3

GHG emissions (only CH4 observed, emission in kg CO2 eq. m�3) during

storage of untreated and anaerobically digested slurry with and without

starch)

Substrate CM-0 CM-29 CM-56 MIX-29 MIX-56

HRT (d) 0 29 56 29 56

Storage duration (d) 105 75 55 118 140

4 8C 0.1 0 0 0.5 0.3

11 8C 18.0 0.1 0 1.20 0.9

20 8C 31.5 2.9 0.2 13.8 11.2

30 8C 36.0 15.0 2.4 19.5 20.2

HRT: hydraulic retention time, CM: cattle slurry, MIX: mixture of manure

and potato starch.

CH4 emissions from digested slurry with or without a straw

cover on the slurry surface. A wooden cover, however,

significantly reduced CH4 emissions from both untreated

and digested slurry. Cumulated NH3 emissions increased

linearly throughout the storage period. Covering the tank

with a wooden lid decreased NH3 emissions from untreated

slurry. In contrast to CH4, a layer of chopped straw on the

surface of digested slurry significantly reduced NH3

emissions. A wooden cover had no additional mitigation

effect. Cumulated N2O emissions increased steadily during

the 100-day measurement period with no significant

difference between treatments. They ranged from 38.2 to

44.0 g N2O m�3.

Total GHG emissions during winter storage were highest

from untreated, uncovered slurry. A wooden cover con-

siderably reduced emissions of the three gases and, hence,

total GHG emissions. Digested slurry emitted less GHG than

uncovered untreated slurry. The combination of chopped

straw and lid reduced total GHG emissions from digested

slurry (Table 4).

During the 140-day storage under warm summer

conditions, considerably more CH4 was emitted than under

cold winter conditions. Cumulated CH4 emissions followed

a quadratic curve and declined towards the end of the

measurement period (data not shown). Untreated slurry

emitted significantly more CH4 than digested slurry. Similar

to winter conditions, a wooden lid reduced CH4 emissions of

untreated slurry. After digestion, CH4 emissions from

uncovered and straw covered slurry were similar, whereas

they were reduced by an additional wooden lid. In summer,

Table 4

Cumulated CH4, NH3, N2O, and greenhouse gas emissions during a winter (100

Treatment Winter experiment

CH4

(g m�3)

NH3

(g m�3)

N2O

(g m�3)

GHG

(kg CO2 eq

Untreated_crust 164.3 a 72.5 a 44.0 a 17.1

Untreated_cover 142.0 b 52.2 b 38.2 c 14.8

Biogas 111.3 c 62.0 c 40.1 b 14.8

Biogas_straw 114.5 c 49.6 b 39.9 b 14.8

Biogas_straw_cover 81.1 d 48.7 b 40.7 b 14.3

Different letters within columns indicate significant differences at p < 0.05.

uncovered digested slurry showed the highest NH3 emis-

sions. They could be reduced by a layer of chopped straw,

and even further by a layer of chopped straw and a wooden

lid. Cumulated N2O emissions showed a linear increase

throughout the measurement period (Amon et al., 2004).

Covering of untreated slurry with a wooden lid increased

N2O emissions, whereas the combination of chopped straw

and a wooden lid decreased N2O emissions from digested

slurry.

During summer storage, total GHG emissions from

untreated slurry were nearly twice as high as from digested

slurry. A wooden cover reduced GHG emissions for both

substrates. As in winter, a layer of chopped straw alone did

not mitigate GHG emissions from digested slurry.

The overall GHG emissions were three to five times

higher in summer compared to the winter experiment.

During summer, NH3 and N2O emissions increased by a

factor of <4 and <2, respectively. But CH4 emissions were

at least 10 times higher during summer compared to the

winter experiment (Table 4).

3.3. Field application

Cumulated NH3 emissions after field application are

shown in Fig. 1. For all substrates emissions were highest

during the first 4 h and reached 73–86% of total emissions

during the first day after application. Only minor emissions

were found after 2 days.

Total NH3 emissions ranged from 0.3 to 1.4 g NH3–N

m�2, representing 5–23% of the NH4–N applied in slurry.

Because of the high variability among replicate plots, no

significant differences between treatments were found.

Substantial CH4 emissions were observed only for a short

time after substrate application (data not shown). Within the

first 4 h after application more than 90% of the total CH4 was

emitted, and after 2 days, CH4 emissions from slurry

treatments did not differ from control plots. Total CH4

emissions from untreated cattle slurry were highest. No

significant differences were observed between untreated and

digested substrates.

During the first day, all fertilised plots showed an increase

in N2O emissions (Fig. 2a), but no significant differences in

cumulated emissions were observed between fertilized and

control plots after a week. At the first rainfall 10 days after

-day) and summer (140-day) storage experiment

Summer experiment

. m�3)

CH4

(g m�3)

NH3

(g m�3)

N2O

(g m�3)

GHG

(kg CO2 eq. m�3)

3591.2 a 110.5 a 48.7 a 90.5

2999.0 b 60.0 b 58.6 b 81.1

1154.2 c 222.5 c 72.4 c 46.7

1191.9 c 125.7 a 75.7 c 48.5

1021.4 d 78.1 d 61.4 b 40.5


Fig. 1. Cumulated NH3 emissions within 4 days after application (60 kg

NH4–N ha�1). Error bars show standard deviation (n = 4).

application, a significant N2O peak occurred in the CAN plot

only. Nitrous oxide emission from the CAN plots remained

highest throughout the year, whereas there were no

differences between the plots receiving organic fertilizers.

Over time an equalisation of all plots took place, and due to a

high variability, no significant differences in N2O emissions

Fig. 2. (a) N2O emissions rates within the first week after application of

mineral fertilizer (CAN), digested and undigested slurry and digested

mixtures and (b) cumulated N2O emissions within 1 year after application.

Error bars show standard deviation (n = 4).

from the different treatments were observed after 1 year

(Fig. 2b).

The cumulated GHG emissions from the organic

substrates after field application ranged from 18.6 to

27.4 g CO2 eq. m�2 a�1, corresponding to 4.14–8.12 kg

CO2 eq. m�3 a�1 substrate. Due to the high variability of

emissions, no treatment effects could be detected (Table 5).

For all organic substrates, N2O represented the highest share

of GHG emissions (74–83%), followed by NH3 (15–24%)

and CH4 (2–3%). The significantly higher CH4 emissions

from untreated slurry shortly after field application had no

effect on total GHG emissions.

4. Discussion

The fermentation studies showed that additives may

significantly increase biogas production (Table 2), but an

increase of CH4 emissions during the storage of digested

slurry was also observed (Table 3). Fermentation did not

change substrate characteristics in a way that higher amounts

of total GHG were emitted after field application (Table 5).

The use of additives, which is often used to enhance the

profitability of biogas plants, should be applied with due

attention to control of GHG emissions during storage.

The addition of renewable substrates such as starch,maize,

etc. instead of urban or industrial waste products minimizes

the risk of contamination with, e.g. heavy metals. But in

addition, the overall nutrient concentrations in the mixtures

may be higher as compared to slurry alone. As a consequence,

concentrations of N, P, and K should be known before field

application, since a higher acreage may be necessary for

proper utilization of the nutrient enriched substrates.

Under summer conditions, considerably more CH4 was

emitted from slurry stores than under winter conditions

(Table 4). Total CH4 emissions in summer were in the same

order of magnitude as those measured by other authors at

similar temperatures (Kulling et al., 2002; Sommer et al.,

2000;Amon et al., 2002). The seasonal variation is in linewith

the known temperature sensitivity of CH4 production (Amon,

1999; Sommer et al., 2000). Untreated slurry emitted more

CH4 during storage than digested slurry. During anaerobic

digestion, organic matter is degraded to CH4 and CO2,

resulting in a residuewith a lower dry matter content and thus

a lower potential for CH4 formation. This finding is in line

with results fromdairy cattle slurry stores (Amon et al., 2002).

However, Sommer et al. (2000) observed higher CH4

emissions from digested than from untreated slurry. In their

experiments, the HRT of slurry digestion was only 10–12

days, which is too short for a complete degradation of

fermentable organic matter. When Amon et al. (2002)

measured CH4 emissions from digested slurry that had not

fully been digested in the biogas plant, they as well observed

an increase in CH4 emissions compared to fully digested

slurry. However, even with a sufficiently long HRT, CH4 is

still produced after anaerobic digestion. It has thus


Table 5

GHG emissions after field application of digested slurry

N2O emission (1 year)

(mg N2O–N m�2)

NH3 emission (4 days)

(mg NH3–N m�2)

CH4 emission (4 days)

(mg CH4–C m�2)

CO2 equivalents

(g CO2 m�2 a�1)

CO2 equivalents

(kg CO2 m�3 a�1)

Control 28.0 (12.38) 0.8 (0.83) 13.7

CAN 58.6 (28.16) 298 (344) 1.9 (1.37) 30.0

CS-0 40.7 (11.30) 711 (475) 27.1 (6.97) 24.0 4.2

CS-29 42.7 (16.32) 797 (889) 16.1 (3.28) 25.1 5.9

MIX-29 41.6 (10.52) 1385 (761) 15.2 (4.16) 27.4 8.1

MIX-56 29.5 (12.33) 768 (334) 20.6 (2.79) 18.6 5.1

CAN: mineral fertilizer; CS: cattle slurry; MIX: digested cattle slurry and potato starch; numbers: 0, 29, 56: days of treatment in the biogas plant; in brackets:

standard deviation; (n = 4).

recommendable to include all stores of biogas plants in the gas

bearing system for complete collection of CH4 and optimum

environmental and economic benefit.

A wooden cover significantly reduced CH4 emissions

from untreated and digested slurry. A cover may shelter the

natural surface crust from rain and help to keep it dry during

winter, but also help to prevent excessive drying during

summer.Methane oxidation has been suggested to take place

in surface crusts (Sommer et al., 2000), and Petersen (2004)

recently provided direct evidence for the presence and

activity of methanotrophs in this environment.

Addition of a wooden cover reduced NH3 emissions from

untreated slurry compared to emissions with a natural

surface crust alone. This is in line with, e.g. De Bode (1991)

and UN/ECE (2002). In winter, NH3 losses from digested

slurry were similar to those from untreated slurry. In

summer, NH3 emissions from biogas slurry were twice as

high as those from untreated slurry. Whereas winter

emissions were at a low level with all treatments, the warm

weather during summer storage enhanced NH3 emissions

especially from digested slurry. Denmead et al. (1982)

developed a regression equation that estimates NH3 losses

from NH4–N content, temperature, and pH. They showed

that a rise in temperature results in higher NH3 emissions

especially if slurry NH4–N content and pH are high. With a

layer of chopped straw, NH3 emissions were nearly as high

as from uncovered, untreated slurry. A wooden cover

reduced NH3 emissions by 65% compared to uncovered

digested slurry. Hence, mitigation of NH3 emissions is

another reason to cover slurry storages at biogas plants.

No distinct differences in N2O emissions between

summer and winter conditions were observed. This is in

line with Sommer et al. (2000), who did not find an influence

of temperature on the level of N2O emissions. Nitrous oxide

is produced from nitrification of NH4 in aerobic zones and

from subsequent denitrification in anaerobic zones. Petersen

(2004) showed that nitrite and nitrate was present in both the

natural surface crust and the straw layer of the same pilot-

scale stores investigated in the present study. Sommer et al.

(2000) found N2O emissions from slurry stores only during

drying conditions, i.e. when the water balance was negative.

In the present study, digested slurry emitted more N2O than

untreated slurry during summer. This might be due to

differences in NH4–N content or structure of the two

materials influencing the potential for nitrification and

denitrification.

Nitrous oxide emissions will be eliminated by gas-tight

cover because the headspace contains no oxygen, a

prerequisite for N2O formation. Stores with a gas-tight

cover can be built on farms with a biogas plant with the

additional advantage that CH4 formed during storage can be

used for electricity production.

Net total GHG emissions were always lower when a

wooden lid was placed on the slurry tank, while a layer of

chopped straw did not reduce GHG emissions. The

reduction was 13.5% (untr_cover), and 3.4% (biogas_-

straw_cover) during winter storage, and 10.4% (untr_cover),

and 3.4 % (biogas_straw_cover) during summer storage.

The NH3 emissions after field application were relatively

low as compared to other studies (Vandre et al., 1997; Wulf

et al., 2002a). This may be due to the trail hose application

we used in this study. It is a technique that reduces NH3

emissions compared to, e.g. broadcast application because

the slurry has an improved soil surface contact.

The temporal dynamics of CH4 emissions were in line

with other studies by Flessa and Beese (2000) and Sommer

et al. (1996). No further CH4 production occurred within the

slurry band after application. This may be due to dry and

cold conditions during the first week of the experiment when

temperatures fell below 0 8C during night time. Other field

experiments under warmer and wet soil conditions indicated

a CH4 production from surface-applied undigested slurry for

up to 4 days (Wulf et al., 2002b). The higher CH4 emissions

from untreated slurry can be ascribed to the higher CH4

production during storage.

Nitrous oxide emissions ranged from 290 to 407 g N2O–

N ha�1 a�1. This corresponds to 0.5–0.7% of the applied

NH4–N and is in line with other studies (Petersen, 1999;

Wulf et al., 2002b). In contrast to results from a laboratory

study (Clemens and Huschka, 2001), there were no

significant differences between untreated and digested

slurry. The high N2O emissions from CAN were probably

due to the higher mineral N-input because in CAN 50% of

the mineral N is in the form of nitrate. Thus, an application

of 60 kg NH4+–N resulted in an overall input of 120 kg

mineral N ha�1, whereas in the slurry treatments, an


application of 60 kg NH4+–N results in an additional input of

partly stabilized organic N, and not nitrate.

5. Conclusions

Anaerobic digestion has a high potential to mitigate GHG

emissions from cattle slurry. Co-digestion of slurry with

additives such as waste starch results in a much higher gas

yield, but the HRT must be sufficiently long to exploit the

potential for gas production without increasing GHG

emissions during subsequent storage and field application.

In the present study,GHGemissions fromopen storeswere

more important than emissions after field application. A layer

of chopped strawwas no reliablemeasure to reduceGHG and

NH3 emissions during slurry storage. In biogas plants, stores

should have a gas-tight cover because CH4 can be collected

for electricity production, and no NH3 and N2O can be

emitted. With a gas-tight cover, GHG emissions will then

mainly occur after field application. In this study, GHG

emissions from untreated and digested slurry after field

applicationwere relatively low and not significantly different.

Biogas production is a very efficient way to reduce the

GHG emissions both through production of renewable

energy and through avoidance of uncontrolled GHG

emissions into the atmosphere during manure management.

Acknowledgement

The studies were financially supported by the EU (EVK2-

CT-2000-00096 (MIDAIR).

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