Bioresource Technology 95 (2004) 235–244
Review paper
Greenhouse gas emission during storage of pig manure
on a pilot scale
Martin Wolter, Shafiq Prayitno, Frank Schuchardt
*
Institute of Technology and Biosystems Engineering, Federal Agricultural Research Centre (FAL), Bundesalle 50, 38116 Braunschweig, Germany
Received in revised form 30 January 2003; accepted 30 January 2003
Available online 2 April 2004
Abstract
The greenhouse gas emissions (CO2 , CH4 , N2 O) from a 2 ton (4.4 m3 ) deep litter pig manure pile (storage time 113 days during
winter season) were quantified by using a tent, which covered the whole pile during the measuring periods only. The emissions were
calculated in CO2 equivalents per kilogram dry matter by. Additionally the retention time (use of tracer gas SF6 ) and the concentrations of the gases in different parts of the pile were determined. The average retention time of the gases in the pile was less than
2 h. CH4 is assumed to have been generated only in the centre of the pile, whereas CO2 was assumed to have been generated in a
wider zone. The emissions of CH4 , CO2 and N2 O were at the highest in the beginning when nearly the whole pile had temperatures in
the range of thermophilic microorganisms. This leads to the assumption that mainly thermophilic microorganisms formed the gases.
The most important gas for global warming was found to be nitrous oxide.
2004 Elsevier Ltd. All rights reserved.
Keywords: Pig manure; Methane; Nitrous oxide; Ammonia; Greenhouse gas; Storage
1. Introduction
Anthropogenic activities like agriculture contribute
to the increase of greenhouse gas concentration in the
atmosphere, resulting in a higher global warming potential (Ahlgrimm, 1995). The greenhouse effect of CH4
and N2 O can be determined by specific global warming
potential (Houghton et al., 2001). It is an index for
estimating the comparison (in kg kg1 ) between the relative global warming contribution due to an atmospheric emission of a particular greenhouse gas, and the
emission of carbon dioxide (CO2 ) for a certain time
horizon.
Apart from emitting the greenhouse gases methane
(CH4 ) and nitrous oxide (N2 O), agriculture is furthermore known as the most important source for ammonia
(NH3 ) emission in Europe (Sommer and Hutchings,
2001). Nevertheless, NH3 has only an indirect global
warming potential, which has been shown in previous
studies to be negligibly low (Wolter et al., 2002). Germany has committed herself to reducing the so-called
Kyoto gases by 21% between 1990 and 2008/2012.
*
Corresponding author. Tel.: +49-531-5964126.
E-mail address: [email protected] (F. Schuchardt).
0960-8524/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2003.01.003
Hence, it is expected that the quantification of greenhouse gas contribution from agriculture has to be done
more accurately (D€
ohler et al., 2002). Important sources
of emissions include the storage of solid manure (Berges
and Crutzen, 1996; Clemens and Ahlgrimm, 2001).
In manure, urea is rapidly converted into CO2 and
NH3 (Beline et al., 1998). NH3 then can be metabolized
under aerobic conditions to nitrate (NO
3 ). Under conditions of low O2 , pressure NO
can
be
denitrified
to N2
3
(Groenestein et al., 1993; Schmidt et al., 1999). From
both nitrification and denitrification processes, N2 O can
be emitted. CH4 is formed by a group of archaebacteria
under anaerobic conditions. Both aerobic and anoxic
conditions are found in the different parts of manure
piles. The greenhouse gas emissions from these processes have been examined in some studies (Martins and
Dewes, 1992; Hellmann, 1995; Hao et al., 2001; Sommer, 2001). High emissions of N2 O have been obtained
from pig manure (Petersen et al., 1998) which contains a
much higher total Kjeldahl nitrogen (TKN) than cattle
manure. Also the sum of NH3 -N and NHþ
4 -N, which is
given as total ammoniacal nitrogen (TAN), is higher.
Most published experiments were carried out using
the cover box method. This is a simple method, but has
the disadvantage that it determines the emissions only
236
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
from a part of the investigated pile. Due to the wide
spatial variability in the emissions, a more accurate
quantification by using a method, which covers a whole
pile, would be more appropriate as was done by Amon
(1998) and Osada et al. (2001). In some studies, gas
concentration inside a pile was measured in order to
obtain some information about gas formation (Hao
et al., 2001; Czepiel et al., 1996; Petersen et al., 1998).
Because the velocity of gas flow inside a heap has not
been directly measured, there are some uncertainties in
deducing the rate of gas formation from its concentration. For the calculation of gas formation from the
concentration inside the pile, a measurement of the
retention time would provide valuable information.
The aims of this investigation were to measure
quantitatively the emissions coming from the storage of
deep litter pig manure in more practical scale (4.3 m3 )
than in former trials in laboratory scale (Huether, 1999;
Wolter et al., 2002) and to obtain information on gas
formation and the retention time of the gases in the pile.
2. Methods
Manure from a deep litter fattening pig house in FAL
was stored outside during autumn and winter over 113
days (23.10.2001–13.02.2002). The litter was short straw
of about 5–10 cm length. The initial geometrical shape
Height (m)
1.6
(a)
1.2
1.6
3
Initial Shape
1.2
4
5
1
0.4
0.8
L3
0.4
Final Shape
0.0
U2
U1
L2
L1
Final Shape
V2
V1
0.0
0.0
2.8
Width (m)
(c)
Initial Shape
2
0.8
of the pile was a conical frustum of 1.3 m height, 2.8 m
diameter of the bottom and 1.2 m diameter of the top
area (Fig. 1). The initial volume was 4.5 m3 . The geometrical shape was measured weekly by taking the radius of the cross sectional area in 0.1 m height intervals.
Cross sectional temperature profile was measured
weekly in 0.2 m cross sectional grid by using a type GTF
NiCr–Ni sensor (Fig. 1a). The manure was analysed
both at the beginning of the storage by randomly taking
12 samples from the pile. At the end, the analysis was
based on 24 samples taken from the pile in concentric
rings with radii of 0 m (centre), 0.56 and 1.12 m in layers
each of 0.2 m height.
The leachate was sucked away and analysed for dry
matter (DM), total Kjeldahl nitrogen and total carbon
content. Gas samples from the inside of the pile were
taken at least weekly through Perspex tubes (outer
diameter: 30 mm, inner diameter: 26 mm), of which two
were inserted horizontally (at a height of 0.4 m, L1, L2
and L3V1; or 0.8 m, U1 and U2 in Fig. 1c and d) and
one vertically into the centre of the manure (V1 and V2
in Fig. 1c and d). The holes on the tubes of 5 mm
diameter, located at 0.4 m distance from one to the
other, allowed the passage of manure gas into a number
of chambers inside the tube, which were connected by
metal pipes for sample taking. Due to the geometrical
change of the heap over the period of storage, the position of the horizontally inserted perspex tubes,
0.4
0.8
1.2
1.6
2.0
2.4
2.8
0.0
2.8
(b)
2.4
2.4
2.0
2.0
1
1.6
2
3
4
5
0.4
0.8
1.2
U2
U1
1.2
L3
L2
L1
0.8
0.8
0.4
0.4
0.0
0.4
0.8
2.0
2.4
2.8
(d)
1.6
1.2
1.6
V1.V2
0.0
0.0
0.4
0.8
1.2
1.6
Width (m)
2.0
2.4
2.8
0.0
1.2
1.6
2.0
2.4
2.8
Width (m)
Fig. 1. Manure pile with initial and final shape and measuring points: (a) vertical section with temperature measurement points and positions of
cover box measurements (1; . . . ; 5); (b) same as (a), cross section; (c) vertical section with gas sample positions inside the pile; (d) same as (c), cross
section; arrows indicate the movement by change of the shape.
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
Fig. 2. Manure pile with tent for measuring the gas emissions.
unavoidably also changed. During the first 20 days it
was from 0.8 to 0.6 m for the upper one and from 0.4 to
0.3 m for the lower one. After 20 days the tubes moved
only slightly.
The emissions were detected by covering the total pile
periodically with a tent (metal frame with polyethylene
foil; 2.9 · 2.9 · 2.5 m3 , Fig. 2). Outside the measuring
periods the pile was open to the natural atmospheric
conditions. After the installation of the tent, the concentration of greenhouse gases was measured at 0, 10, 20
and 30 min. The emissions were calculated by subtraction of final concentration from initial concentration of
each 10-min interval. For the calculation of the total
emissions, only the values measured during the first 10
min of the observation were used. The values taken from
the second and third 10 min of the measurement were
meant for control. The reason was because the heat and
mass transfer between pile and ambient air is disturbed
less in a shorter enclosure time. During the measurement
time, the air inside the tent was ventilated with two fans
(30 W, diameter of rotor 23 cm). The emissions were
calculated as the product of concentration change and
volume of the free air space in the tent. Samples were
taken at least twice a week during the first 56 days and
once a week beyond that time. The gas losses, which
eventually occurred from the tent during the measurement, were checked at the first four measurements by
the injection of each 1 ml SF6 (sulfur hexafluoride) into
the tent before the measurements started.
The emissions at different regions of the pile surface
were detected by a cover box with a diameter of 22 cm
and a volume of 3.96 l and were calculated by subtraction of the final from the initial concentrations in the
box between 0 and 2 min. A short measurement time
was chosen because with longer duration, the dynamic
gas flow between the pile and ambient air would be
disturbed more. For calculating the emissions from
237
different surface areas, the pile was divided into different
surface areas (Fig. 1a and b). The lower surface side area
was assumed to be from the bottom up to half of the
height of the pile (cover boxes 1 and 5) and the upper
side area was assumed to be the upper half (cover boxes
2 and 4). A plane top surface of 0.6 m in radius was
assumed to be horizontally on the top and calculated
separately (cover box 3). In each of the lower side and
upper side areas, two measurement points were selected,
with one for the horizontal top layer.
The retention time of gases in the pile was estimated
by firstly injecting 10 ml of SF6 through a metal tube
with outer diameter of 3 mm and inner diameter of 1
mm into the pile at 0.4 m in height and 1.4 m in depth,
then secondly by measuring the emissions using the tent
as described above.
The concentrations of CH4 , N2 O, CO2 , O2 and SF6
were determined by a gas chromatographic system
(Shimadzu GC 14B) as described by Huether et al.
(1995). In brief, the system was equipped with two different columns (Porapak QS, 80/100: 1.5 and 3.0 m;
60/80: 4.5 m; 80 C) and three detectors
molsieve 5 A,
(FID: 320 C, ECD: 320 C, TCD: 100 C). The carrier
and the make-up gases for the system were helium and
nitrogen, respectively. The samples were injected by
using a headspace sampler (Dani, HSS 86.50; aux. press.
0.18 bar, carr. press. 2.80 bar).
3. Results
3.1. Temperature
During the first 14 days the temperature in the pile was
between 40 and 74 C except at the outer layer, of
maximum 0.4 m below the surface, which had temperature between 30 and 40 C (Figs. 3 and 4). After that
time, the temperature decreased. The area with the
highest temperature was found in the centre of the pile at
a height of about 0.4 m. After day 43 most of the cross
sectional area was in the range of mesophilic microorganisms. It decreased especially in the outer zones, to
below 10 C. After day 65 in the area up to 0.6 m high at
the bottom radius the reading was below 20 C, while in
the inner area at the height of about 60 cm from the
bottom a maximum of 50 C was measured. At the end of
storage, the maximum temperature was 39 C.
3.2. Changes of physical and chemical properties
During storage, the bulk density increased from 0.44
to 0.61 m ton1 , whereas DM decreased from 36.2% to
24.9% because of rainfall (861 mm) and low water
evaporation during autumn and winter season (Table 1).
Losses of DM in the leachate (6.2 kg in 726 l) were
negligibly low compared to total DM losses of 398 kg by
238
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
Temperature (°C)
Temperature (°C)
Temperature (°C)
Fig. 3. Temperature profiles inside the manure pile (vertical section).
80
U1
60
U2
40
20
0
0
20
40
60
80
100
120
80
L1
60
L2
40
L3
20
0
0
20
40
60
80
100
120
80
V1
60
the respiration process. The contents of TKN and TAN
changed from 33.7 to 37.8 g kg1 DM and from 11.9 to
6.4 g kg1 DM, respectively. At the end of the retention
time the NO
3 content was eight times and the NO2
content four times higher compared to the start. The
analyses of DM and TKN show a wide variety and the
inhomogeneous composition of the manure after storage
(Table 2). However, a strong linear correlation was
found between DM and TKN (r2 ¼ 0:87), which can be
explained by spatial differences in the straw content. The
pH was at the highest in the centre at the height of up to
0.6 m with values between 9.2 and 9.3. It decreased to
values between 7.7 and 8.6 directly under the surface.
For both initial and final samples, high variability of
values was found resulted from the heterogeneity of the
material. For instance the values for initial DM varied
between 19% and 72%.
V2
40
3.3. Retention time
20
0
0
20
40
60
80
Time (days)
100
120
Fig. 4. Temperature course at gas sample measurement points Fig. 1;
ambient temperature: )9.7 to 12.3 C, average ambient temperature:
5 C.
The use of SF6 allowed a direct measurement of the
average retention time of the gases inside of the pile.
This measurement was performed at day 79, by injecting
10 ml of SF6 into the centre of the pile at the height of
0.4 m. As much as 64% of the injected SF6 was emitted
during the first 2 h after the injection. Four days after
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
239
Table 1
Composition and mass balance of deep litter pig manure before and after storage for 113 days
Initial
Final
Mass
(ton)
DM (%)
ODM
TKN
0.44
0.61
2.0
1.6
36.2
24.9
840
755
33.7
37.8
(kg)
(kg)
(kg)
(kg)
(kg)
(kg)
(kg)
(kg)
(kg)
2.00
1.60
)400
)20
724
398
)326
)45
608
301
)307
)51
24.4
15.1
)9.4
)38
8.6
2.6
)6.0
)70
0.58
2.56
+1.98
+341
0.08
0.16
+0.08
+110
25.1
17.8
)7.3
)29
264
144
)120
)46
Mass
Initial
Final
Diff. abs.
Diff. in %
Composition of DM (g kg1 )
BD
(m3 ton1 )
–
–
–
pH (–)
TAN
NO
3
NO
2
Ntot
C
C/N
11.9
6.4
0.8
6.4
0.1
0.4
34.6
44.6
365
362
11
10
8.6
8.3
–
–
–
–
–
–
–
–
BD: bulk density.
Initial: average of 12 samples; final: average of 24 samples.
Table 2
Properties of deep litter pig manure after storage for 113 days at different measurement points
Height (m)
Radius (m)
TKN (g kg1 DM)
0.6–0.8
0.4–0.6
0.2–0.4
0.0–0.2
0
0.56
2.8
3.0
3.5
1.4
1.8
4.9
1.9
DM (%)
pH (–)
1.12
0
0.56
2.6
1.0
28.3
28.5
26.6
21.4
20.5
34.7
21.2
the injection (5800 min), the concentration of the injected SF6 in the gas inside the pile was less than 0.5%
(Fig. 5). These values are confirmed by a preliminary
investigation with deep litter pig manure (bulk density
0.63 ton m3 ) using a tent roof for protection against sun
and rain. The SF6 emission after injection at days 57 and
79 into the pile at 0.2 m height above bottom was
measured each 30 min with a cover box on top of the
pile. Maximum SF6 emission rates at day 57 were 90 min
and at day 79, 120 min after injection. These findings
suggest that generally most of the gas inside a pile would
be emitted within 2 h during the first 80 days of storage
of deep litter pig manure.
1.12
0
0.56
1.12
30.0
17.3
7.7
9.2
9.3
9.2
8.3
8.3
8.5
8.4
8.6
This, together with the emission rates found, provides
information about areas with gas formation. By considering the existence of O2 , CO2 and CH4 the pile can
be divided into four different zones, each of them with
its own characteristics (Fig. 7, Table 3). Zone I, about
92% of the total volume of initial shape, is the zone with
largely aerobic conditions during whole rotting time and
low emissions of the greenhouse gases CH4 and N2 O.
Zones II and III (7.7% of total volume) have an oxygen
lack most of the rotting time, that means aerobic and
anaerobic conditions with low CH4 and high N2 O
emissions. Zone IV (only 0.3% of total volume) is a
permanently anaerobic zone with high CH4 emissions
and very low N2 O emissions.
3.4. Gas composition inside the pile
The composition of gas inside the pile (Fig. 6) is an
indicator for aerobic and anaerobic rotting processes.
SF6 in the pile (%)
100
80
60
40
4 days
20
0
1
10
100
1000 10000
Time after injection (minutes)
Fig. 5. Remaining SF6 inside the manure pile in percent of total injected tracer gas (10 ml) at storage day 79.
3.5. Emissions from different surface areas
The measurements of the emissions from different
surface areas were done with a cover box at three different heights of the pile (Fig. 1a and b). They indicated
that almost all of the gases CH4 , CO2 and N2 O were
emitted through the horizontal top surface of the pile
(cover box 3) and the upper half of the side (cover boxes
2 and 4), and only a small part through the bottom half
of the side (cover boxes 1 and 5) (Table 4).
3.6. Emission course from the whole pile
The measurement of the emission course from the
whole pile was conducted by using the specially designed
240
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
O2 (%)
25
20
15
10
5
0
CO2 (%)
60
U1
U2
40
U1
U2
0
25
20
15
10
5
0
0
25
20
15
10
5
0
0
20
40
60
80
100
20
120
600
800
600
400
200
0
0
800
600
400
200
0
0
40
60
80
100
L1
L2
L3
20
40
60
80
100
120
V1
V2
120
L1
L2
L3
40
20
0
0
20
40
60
80
100
60
120
V1
V2
40
20
20
40
60
80
100
120
U1
U2
20
0
0
20
40
60
80
100
120
CH4 (%)
400
200
0
0
20
60
N2O (ppm)
800
0
0
40
60
80
100
120
80
60
40
20
0
0
U1
U2
20
40
60
80
100
120
80
L1
L2
L3
L1
L2
L3
60
40
20
20
40
60
80
100
120
0
0
120
80
60
40
20
0
0
V1
V2
20
40
60
80
100
Time (days)
20
40
60
80
100
120
V1
V2
20
40
60
80
100
120
Time (days)
Fig. 6. Gas composition inside the manure pile in percent of the total gas volume (U, L and V are measuring points as on Fig. 1).
tent. The accuracy of the methodology using the tent
was checked during the first four measurements by the
injection of SF6 into the air space in the tent before
measurement started. It was meant to detect the eventually occurring losses of gas during measurement. The
average loss was 8% during the first 10 min and 16%
during the whole 30 min of measurements. A movement
of SF6 into the pile could explain these losses, where the
volume of the pile itself accounted for approximately
20% of the volume of the whole air space inside the tent.
At day 22, the temporal variability of emissions was
measured during 5 h (five times, each 1–2 h interval).
The relative SD for CH4 , N2 O and CO2 was less than
11% indicating that the emission rate was relatively
constant for these gases and that the measurements can
be regarded as representative.
The gaseous emissions from the whole surface were at
the highest at the beginning of the storage (Fig. 8). The
CH4 -C emissions increased from 26 mg kg1 (DM) d1
on the first day to 63 mg kg1 (DM) d1 on day 27, and
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
241
3.7. Carbon and nitrogen balance
1.6
Height (m)
(a)
Initial Shape
1.2
I
II III II
0.8
Final Shape
0.4
0.0
0.0
Of the carbon, 20.5% was respired as CO2 and 55.3%
remained in the manure (Table 5). Practically no carbon
was found in the leachate and 2% of the carbon was
emitted as CH4 . The initial total nitrogen content of the
manure contained 35% as total ammonia nitrogen,
which is the dominating compound for mineral nitrogen. During the composting or storage, a part of the
mineral nitrogen was rebound into organic compounds
or denitrified. Consequently after storage, the TAN
content was reduced to 54% of the initial concentration.
Only a negligible part was found in the leachate and was
emitted as N2 O. The calculated loss of nitrogen by
denitrification and ammonia emission was 26.3%, which
is lower than values from Petersen et al. (1998) who
obtained 57% loss for pig manure stored during autumn.
Calculated on the analyses results of the manure at the
start and at the end the gaseous nitrogen losses were
about 29% (Table 1).
I
IV
0.4
0.8
1.2
1.6
2.0
2.4
2.8
Width (m)
(b)
I
III.IV
II
Fig. 7. Zones in the manure pile: (a) vertical section, (b) cross section.
3.8. Effect of emissions on global warming
decreased strongly to less than 6 mg kg1 (DM) d1 on
day 50. The highest CO2 -C emissions were at the first day
with 2811 mg kg1 (DM) d1 and decreased to less than
700 mg kg1 (DM) d1 after day 36. The highest N2 O-N
emissions were at day 11 (28 mg kg1 (DM) d1 ) and
decreased to less than 6 mg kg1 (DM) d1 after day 40.
The main originator for global warming during
manure storage was N2 O, even only 1.9% of total
nitrogen emitted as N2 O. It contributed up to 78.4%
(307.7 g kg1 DM) to the CO2 equivalent emissions,
while methane contributed only 21.6% (84.8 g kg1 ). The
total CO2 equivalent emissions were 392.5 g kg1 DM.
The gaseous nitrogen losses (6.6 kg NH3 and N2 , see
Table 5) are unaccounted for that calculation.
Table 3
Characteristics of different zones in the manure heap
Zone
MP
O2
I
L2, L3, U2
Whole time: >15%
At start: max. 6%
Whole time: <0.5%
Characteristics: permanent aerobic zone with traces of CH4 and N2 O
II
L1, U1
Start to day 50: <5% at L1 Start to day 50: 20–22%
Whole time: <3%
After 70 and 80 days resp.:
<15% at U1
peak up to 647 ppm
Characteristics: zone with oxygen lack during the first seven weeks and low CH4 and high N2 O emissions
III
V2
Start to day 80: <10%
IV
V1
Whole time: <5%
Start to day 98: 44–16%
Day 14–43: 29–60%
Characteristics: permanent anaerobic zone with high CH4 emissions
CO2
CH4
Start to day 80: 5–20%
N2 O
Whole time traces
Whole time: <1%
After 80 days: peak up to
600 ppm
Characteristics: zone with permanent oxygen lack during the first 12 weeks and low CH4 and high N2 O emissions
Whole time traces
MP: measuring points (Fig. 1).
Table 4
Emissions from different surface areas of the manure pile in percentage of total emissions obtained by cover box measurement
Area
Cover box (no.)
Top surface
Upper half of the side surface
Bottom half of the side surface
3
2+4
1+5
Total
CO2 (%)
CH4 (%)
N2 O (%)
67
26
7
73
21
6
52
41
7
100
100
100
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
2500
Model: Gauss
Chi^2 = 37.5
y0
2.22
xc
20.3
w
23.2
60
50
CO2-C [mg kg-1 (DM) day-1]
CH4-C [mg kg-1 (DM) day-1]
70
±1.75
±0.72
±1.89
40
30
20
10
0
0
20
40
60
80
100
Y =2888-91.5 X+1.023 X2-0.0037 X3
R2= 0.89
2000
1500
1000
500
0
0
20
Time (days)
40
60
80
Time (days)
25
NO2-N [mg kg-1 (DM) day-1]
242
Y =21.5-0.520 X+0.00266 X2+8.21E-6 X3
2
R =0.70
20
15
10
100
5
0
0
20
40
60
80
Time (days)
100
Fig. 8. Time course of gaseous emissions of the manure heap.
Table 5
Carbon and nitrogen balance during and storage of pilot scale deep litter pig manure for 113 days in percent of the initial amount
Carbon
(kg)
(%)
Nitrogen
(kg)
(%)
Initial carbon content in manure
Final carbon content in manure
Carbon in leachate
CO2 -C emission
CH4 -C emission
Recovery
264
144
0.04
54.0
2.0
200
100
55.3
0.02
20.5
0.8
75.8
Initial nitrogen content in manure
Final nitrogen content in manure
Nitrogen in leachate
N2 O-N emission
25.1
17.8
0.20
0.48
100
70.9
0.8
1.9
Gaseous loss of N2 , NH3 and
possible measurement error
6.6
26.3
4. Discussion
After the composting period of 113 days, the mass of
TAN accounted for 30% of the initial mass. This value is
in contrast to the study with manure from dairy cows
conducted by Sommer (2001), who found only 4% after
composting for an untreated pile. But it corresponds to
the values published by Webb et al. (2002) during aerobic storage of pig manure (bulk density 0.53 ton m3 )
who found a content of 60% after storage.
We found out that the calculated emissions from the
cover box method were 3–7 times lower for CO2 and
N2 O than those from the tent method. This can be explained by a high spatial variability of gas emissions
through the surface of the pile. Because of the inhomogeneous structure of the manure it was possible that
some ‘hot spots’, like channels with lower bulk density
and higher gas exchange, were not detected by the cover
box method. Therefore, the values from the tent method
were used for the quantification of overall emissions.
The present, typical temperature profile was also
found by Fernandes et al. (1994) investigating compost
piles. Due to the high temperatures in the centre, a
chimney effect is proposed by which air is drawn
through the lower sections into the pile and warm air is
moved upwards resulting in convection aeration.
Together with the emissions, the following conclusions concerning gas formation and gas flow in the pile
can be made: The emission rates of CO2 and CH4 (Fig.
6) correlated linearly (r2 ¼ 0:71 and 0.74, respectively)
with the temperature in the centre at 0.2 m height (Fig.
4, measuring point V1). Obviously the highest activity of
organic carbon mineralization processes occurred during the thermophilic phase and decreased with age and
lower temperatures after the easy degradable compounds were consumed. Because CO2 -C emission was 27
greater than CH4 -C, CO2 must have been formed mainly
by aerobic processes (Table 5). Otherwise, CH4 -C
emissions would have been much higher than CO2 -C
emissions. Both the proposed CH4 formation only in a
small region (Fig. 7, zone IV) and the gas flow from the
centre being upwards were confirmed by a linear correlation between the CH4 concentration in the centre
and the CH4 emitted per day (r2 ¼ 0:88). Because of the
relatively high final pH value in zone IV (Table 2), it can
be suggested that aerobic processes must have occurred
in that zone at the end of incubation after CH4 emissions had declined.
CO2 is assumed to have been formed in zones II–IV
(Figs. 6 and 7) because of the high concentrations found
in those zones. However, a more detailed examination of
gas flow is necessary to justify this assumption.
N2 O concentrations and emissions were less predictable than those of CH4 and CO2 . No correlation was
found between N2 O concentration and temperature or
O2 concentration (Figs. 4 and 6). N2 O emission was
M. Wolter et al. / Bioresource Technology 95 (2004) 235–244
relatively high at the beginning. This corresponds to
other published data. The only difference is that between
day 50 and 60 it did not peak again, as it has been observed in studies using either untreated farmyard manure, artificially prepared pig manure or compost that
was turned twice a week (Petersen et al., 1998; Wolter
et al., 2002; Hellmann et al., 1997). An explanation for
the differences in the emission course of N2 O from other
studies might be the different origin of the manure,
which allowed the adaptation of nitrifying and denitrifying processes for microbial communities in the deep
litter during the storage in the house.
The highest N2 O emissions of present study occurred
at day 10 (Fig. 8), when the temperatures in nearly the
whole cross sectional area were in the thermophilic
range (Figs. 3 and 4). Only low N2 O emissions were
found after day 49 although most of the cross sectional
area during that period was in the mesophilic temperature range, which is considered to be optimum for the
nitrifying and denitrifying microbial communities.
Obviously, N2 O was generated mainly at the beginning
by thermophilic organisms, and this seems to be
underestimated in literature reviews (Hellmann, 1995).
The shift of a high N2 O concentration zone to the centre
of the pile during storage was also found by Czepiel
et al. (1996). The first reason for this shift might have
been a shift in N2 O generation towards the centre. The
second reason might have been a slower gas flow, which
resulted in an accumulation of the N2 O formed. This is
confirmed by lower temperatures at the end of the
storage and by low N2 O emissions with high N2 O concentrations at the same time. However, further detailed
studies that include the repeated measurement of gas
flow are required for a better understanding of this
process.
The emissions, calculated in the present study, can be
compared with the results from Sommer and Moller
(2000) who also used deep litter pig manure with a bulk
density of 0.44 ton m3 and a total mass of 4.3 tons.
Their composting period was during spring and summer
season from 31st of March to the 21st of August. The
temperature course in the pile was comparable and
the emissions of CO2 and CH4 were also high at the
beginning, but decreasing earlier than in the present
study. The total amounts of CH4 (0.2% of initial carbon)
and N2 O emissions (0.8% of initial nitrogen) related to
the initial carbon or nitrogen content are also comparable (Table 5).
The main factors for gas emissions of deep litter pig
manure are the initial bulk density of the manure,
influencing the free air space and gas exchange, and
initial carbon and nitrogen contents as well. Furthermore, it can be concluded that the influence of the
atmospheric temperature is quite low, and mainly the
processes inside the pile determine the dynamics of CH4
and N2 O emissions.
243
The calculated CO2 equivalent emissions for the
storage of pig manure were confirmed additionally by a
preliminary investigation using deep litter pig manure
(see Section 3.3). The emissions were measured by
closing the tent during a measurement time of 30 min.
Of the initial carbon, 0.6% was emitted as CH4 -C and of
the initial nitrogen, 1.7% was emitted as N2 O-N.
Acknowledgements
We thank Heike Horn of the FAL for the excellent
wet chemical analyses. This work was funded by the
Deutsche Forschungsgemeinschaft (DFG Germany)
under contract number Schu-1185-/1-3.
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