Biol Fertil Soils (2012) 48:981–985
DOI 10.1007/s00374-012-0714-1
SHORT COMMUNICATION
The extent of methane (CH4) emissions after fertilisation
of grassland with digestate
B. Dieterich & J. Finnan & P. Frost & S. Gilkinson &
C. Müller
Received: 21 March 2012 / Revised: 13 May 2012 / Accepted: 5 June 2012 / Published online: 23 June 2012
# Springer-Verlag 2012
Abstract Methane (CH4) emissions following surface application of liquid digestate were measured on agricultural
grassland. The total extent of emissions was estimated by
curve fitting and integration. The temporal pattern of the
CH4 efflux was well described by fitting double-phase decay curves ( J ðT Þ ¼ B EXPðiT Þ þ C EXPðjT Þ ).
Total emissions were found to be small in all cases (7.3–
11.5 mg CH4 L−1). From a greenhouse gas perspective, this
level of CH4 emissions represents only a few per cent of the
global warming potential of the nitrous oxide emissions
which may occur following application of the digestate.
Keywords Digestate . Methane . Greenhouse gases . Biogas
B. Dieterich : C. Müller
School of Biology and Environmental Science,
University College Dublin,
Belfield, Dublin 4, Republic of Ireland
J. Finnan
Teagasc,
Oak Park,
Carlow, Republic of Ireland
P. Frost : S. Gilkinson
AFBINI,
Large Park, Hillsborough, Co. Down,
Hillsborough BT26 6DR, UK
C. Müller
Department of Plant Ecology, University of Gießen,
Heinrich Buff-Ring 26,
35392 Giessen, Germany
B. Dieterich (*)
c/o Institut für Pflanzenökologie,
Heinrich Buff-Ring 26,
35392 Giessen, Germany
e-mail: [email protected]
Introduction
Anaerobic digestion (AD) of biomass is a promising way to
generate renewable energy in agriculture, and on-farm AD
has expanded considerably in the recent past, particularly in
some European countries (Braun 2007; Weiland 2006). The
organic residue after digestion, the digestate, is rich in plant
nutrients and may be recycled back to agricultural land as an
organic fertiliser (Braun 2007). Digestate utilisation can be
environmentally beneficial since nutrient cycles can be
closed and the need for mineral fertiliser reduced.
However, being taken from a methanogenic environment
(the AD reactor), digestate may give rise to diffuse emissions of methane (CH4) during storage (Amon et al. 2006)
and after field application. Methane emissions after digestate application measured by closed chambers have been
published by, e.g. Amon et al. (2006) and Wulf et al.
(2002). Amon et al. (2006) found field emissions of about
2 mg CH4 L−1 and Wulf et al. (2002) of around 4 mg CH4
L−1 for digestate applied to the surface of grassland plots.
However, it is not clear whether these figures represent the
full extent of emissions. As can be seen in Wulf et al.
(2002), CH4 efflux is highest directly after application and
then declines to very low levels within approximately a day.
It is not evident whether peak emissions were adequately
captured by the methods employed by Amon et al. (2006)
and Wulf et al. (2002). Outside times of digestate application,
upland soils may act as sinks for atmospheric CH4 due to their
methanotrophic microflora (Hütsch 2001; Kern et al. 2012).
The aim of our investigation was to provide estimates of
the full extent of CH4 emissions from digestate applied to the
soil surface on grassland in Ireland. To enable this assessment,
the first sampling of the CH4 efflux after application was
conducted as quickly as possible, and repeated samplings
were carried out to enable an assessment of the scale of the
decline of the flux with increasing time after application.
982
Biol Fertil Soils (2012) 48:981–985
Gas sampling was carried out by the method of manual closed
chambers (Hutchinson and Mosier 1981). The procedure and
analysis are described in detail by Dieterich (2011). In brief,
frames made from stainless steel were permanently installed in
the ground of grassland plots. The digestate was applied
manually in bands to the soil surface within each frame. For
sampling, a lid was put gas tight on the frame, and the first gas
sample was taken immediately after closure with a syringe
through a rubber septum. The gas mixture within the chamber
was sampled two more times during the time of closure
(Hutchinson and Mosier 1981), and the samples were analysed by gas chromatography (Shimadzu GC-2014). Closure
times of the chambers were kept brief (20 min in general). The
area enclosed by the frame was 0.168 m2.
Following digestate application, the emission rate declined notably even on short timescales. Therefore, the slope
of the linear interpolation between the three samples did not
represent the actual level of emissions at the time of closure.
As our aim was to fit curves of CH4 efflux against time, the
first step was to establish CH4 effluxes corresponding to
particular moments in time. The three measurements were
used to estimate the CH4 efflux at the time that the chamber
was closed (i.e. t00), which is a common approach with
closed chamber measurements (Hutchinson and Mosier
1981). First, we calculated parameters a and b of a
Michaelis–Menten-type saturation curve from the three concentration measurements according to
cðtÞ ¼ c0 þ a t=ðb þ tÞ;
ð1Þ
whereby c(t) is the time-dependent CH 4 concentration
(micrograms CH4 per square meter up to the height of the
ð2Þ
Efflux J was related to the volume of digestate applied
(micrograms CH4 per liter of digestate per minute). The time
T that had elapsed between application of digestate and chamber closure was recorded. Equation 1 did not have a sensible
solution for some measurements taken at low levels of efflux
(i.e. high values of T) because the efflux between the second
and third sample was larger than between the first and the
second sample. In this case, linear regression was used.
Total emissions
Similar to the results of Wulf et al. (2002), CH4 efflux J was
highest immediately following digestate application,
reverted to background levels within approximately a day
and remained low until the next application. Small uptake
fluxes around −20 μg CH4 m−2 h−1 were measured on plots
not treated with digestate. Therefore, background CH4 oxidation represented less than 1 % of the peak values of J
following digestate application and thus was too small to
affect the measurements to any perceivable extent. The full
extent of emissions is then represented by the area obtained
by integration over all times of a curve fitted to the measurements of J(T) between application and subsidence to background. However, the different samples from the plots with
identical amounts of digestate did not represent true replicates since it was not possible to sample those plots at
identical times after application (Fig. 1). Instead, all plots
400
Single-phase fit
Free double-phase fit
Constrained double-phase fit
-1
Gas sampling and flux calculation
J ¼ dc=dt jt¼0 ¼ a=b:
300
-1
The investigation was carried out on permanent grassland
growing on well-drained mineral soil at Teagasc, Oak Park,
Carlow, in the Republic of Ireland (52° 86′N, 6° 54′W). Mean
annual temperature at the site is 9.4 °C, and annual precipitation is 785 mm. The textural class of the soil was a sandy
loam, soil pH was 6.3 and soil organic C content was 2.3 %.
The most common species of grass were Agrostis stolonifera,
Elymus repens and Poa pratensis. The digestate was obtained
from an agricultural digester using cattle slurry as the sole
feedstock. Its mean dry matter concentration is 59.3 gL−1,
total N concentration is 3.6 gL−1, 74 % of dry matter is organic
matter, and pH is 7.9 (Frost and Gilkinson 2010).
Methane emissions were measured repeatedly after application of two different volumes of digestate and on two
different occasions (May 2010, 1.5 and 4.0 Lm−2, and July
2010, 1.2 and 3.6 Lm−2). On both dates, the treatments were
applied to at least four different plots.
chamber), c0 is the ambient concentration of CH4, t is time
(minutes) after closure and a and b are curve parameters.
The efflux J at the time of closing was calculated as
CH4 efflux (µg CH4 L min )
Methodology
200
100
0
0
50
100
150
200
Time after digestate application (T, min)
Fig. 1 Fit of different decay functions to CH4 efflux determined after
surface application of 4 Lm−2 digestate to grassland plots in May
(black crosses). It is evident that the single-phase curve (solid line)
declined too rapidly and therefore did not provide a good fit for some
values of T
Biol Fertil Soils (2012) 48:981–985
Table 1 Time elapsed
between digestate application and chamber closure (T, minutes) and
corresponding fluxes J
(micrograms CH4 per
liter per minute) for the
May application date
Time T
(min)
983
1.5 Lm−2
Efflux
(μg CH4
L−1 min−1)
4.0 Lm−2
Efflux
(μg CH4
L−1 min−1)
645
177
196
112
9
14
7
9
4
3
329
236
109
150
16
15
23
12
8
9
0
5
0
−1
0
−1
11
8
1
1
−0
−0
5
9
13
17
95
99
103
107
170
174
178
182
1,606
1,611
1,620
1,631
later (as illustrated in Fig. 1). Therefore, we used doublephase decay curves of the form
J ðT Þ ¼ B EXPðiT Þ þ C EXPðjT Þ;
ð4Þ
whereby, in analogy to Eq. 3, the first exponential term
represents the quickly declining part of the curve, and the
second term represents the slowly declining part.
For two curves (1.5 Lm−2 in May and 1.2 Lm−2 in July),
the double-phase fit did not converge by itself. The fit for
these two curves was instead obtained by constraining the
parameter values such that B+C (Eq. 4) equal the value of A
as found in the fit of a single-phase function (Eq. 3) to the
same data. Alternative fits by this method were also
obtained for the other two curves (i.e. 4.0 Lm−2 in May
and 3.6 Lm−2 in July) to gain insight into the effect of
constraining the parameters as compared to a free fit. All
curve fitting was carried out with SigmaPlot 11.0 (Systat
Software 2008). The fitted curves (Eq. 4) were integrated
over all times to derive the total CH4 emissions per liter of
digestate according to
CH4 emiss ¼ B=i þ C=j:
that received the same amount of digestate were used to
construct a single curve per date and volume of digestate
application. Thus, there were four curves in total based on
19 data points (3.6 Lm−2, July), 16 data points (1.5 and
4.0 Lm−2, May) or 12 data points (1.2 Lm−2, July). The
fluxes used for fitting the curves for the May application are
detailed in Table 1. First, we attempted to fit single-phase
decay curves of the form
J ðT Þ ¼ A EXPðkT Þ;
ð3Þ
to measured emissions, with J(T) as the CH4 efflux at time T
(micrograms CH4 per liter per minute), A as the maximum
CH4 efflux (micrograms CH4 per liter per minute), k as a
decay constant and T as time (minutes) after application.
These curves showed the rapid initial decline of the CH4
efflux but evidently underestimated emissions in samples
taken subsequently, i.e. around 90 min after application or
ð5Þ
Results and discussion
The fitted parameters of the double-phase decay curves are
summarised in Table 2. All the curves showed a good fit to
the data, as illustrated by high adjusted coefficients of determination (Radj2; Table 2). However, due to the relatively
small data set, the exact shapes of the curves were not overly
well defined. The single-phase decay curves tentatively
fitted to the same data also showed high coefficients of
determination but clearly underestimated total emissions
because they declined too rapidly (Fig. 1). Also, two of
the four double-phase curves could only be calculated by
constraining the parameter values as described above in the
“Methodology” section. For the other two curves, the alternative estimates of total CH4 emissions (Eq. 5) from the free
Table 2 Parameters of double-phase decay curves ( J ðT Þ ¼ B EXPðiT Þ þ C EXPðjT Þ ) fitted to measurements of CH4 efflux from
digestate, and total CH4 emissions (milligrams CH4 per liter digestate) estimated from integration of the same curves
Treatment
1.5
4.0
4.0
1.2
3.6
3.6
Lm−2,
Lm−2,
Lm−2,
Lm−2,
Lm−2,
Lm−2,
May (constrained fit)
May (free fit)
May (constrained alternative fit)
July (constrained fit)
July (free fit)
July (constrained alternative fit)
B (μg L−1)
i (μg L−1 min−1)
C (μg L−1)
j (μg L−1 min−1)
Radj2
Total CH4 emissions
(mg L−1)
1616
546
463
319
324
250
0.23
0.20
0.11
0.10
0.20
0.10
90
149
49
106
89
44
0.02
0.02
0.01
0.02
0.02
0.01
0.95
0.97
0.97
0.95
0.99
0.97
11.5
10.0
9.2
9.6
7.3
7.8
984
vs. the constrained fit were not much different (Table 2).
However, the actual shapes of the curves were influenced
more by constraining the parameters (Table 2, Fig. 1). Thus,
our estimates of total CH4 emissions are robust, but the data
base is too small for further-reaching considerations about
the shape of the efflux decline.
Total CH4 emissions derived from the four measurements altogether varied only between 7.3 and 11.5 mg
CH4 L−1 (Table 2). The quickly declining phase of
Eq. 4, B EXPðiT Þ, probably represents a degassing
process of CH4 dissolved in the liquid, as postulated by Wulf
et al. (2002). The slowly declining phase, C EXPðjTÞ ,
might represent delayed diffusion of CH4 or a low level of
production that persists for a short while after application.
Dieterich (2011) had estimated emissions of 5 mg CH4 L−1
from the same gas samples. This value had been obtained by
fitting single-phase curves to fluxes derived from linear interpolation of concentration measurements over the duration of
closure of the chambers. The present investigation is more
accurate in two important respects, namely the reconstruction
of actual CH4 effluxes at the moment of chamber closure and
the use of double- rather than single-phase decay curves.
Consequently, estimated emissions increased. Our estimates
are also higher than those published by Amon et al. (2006) and
Wulf et al. (2002). Therefore, this analysis shows that it is
necessary to sample CH4 efflux quickly and repeatedly after
digestate application and describe CH4 emissions by doublephase curves to arrive at realistic estimates.
In general, the present investigation confirms the minor
importance of CH4 emissions from applied digestate in the
GHG balance of agricultural grassland, as postulated by
Dieterich (2011). For example, at a hypothetical annual digestate application of 60 m3 ha−1 corresponding to about 200 kg
available N (Frost and Gilkinson 2010), an emission level of
10 mg CH4 L−1 digestate only corresponds to 15 kg CO2eq
ha−1 year−1 (at a global warming potential of CH4 of 25 on a
100-year timeline, Forster et al. 2007). Other agricultural
sources of greenhouse gases, such as nitrous oxide (N2O)
emissions from N fertilisers, produce much greater emissions.
For example, assuming an average N2O–N emission factor of
1 % of applied N (De Klein et al. 2006), application of 200 kg
N year−1 would result in an emission of >900 kg CO2eq (2 kg
N2O–N with a global warming potential of N2O of 298,
Forster et al. 2007). Depending on factors such as environmental conditions and fertiliser type, N2O emissions and
emission factors can vary widely in practice and may even
reach values of greater than 10 % applied N lost as N2O–N
(De Klein et al. 2001; Granli and BØckman 1994).
The difference in the emissions estimated from the different curves (Table 2) indicates the degree of uncertainty
about the true extent of CH4 emissions. In the current work,
the difference between the lowest and the highest estimate
was only 4.2 mg CH4 L−1 (corresponding to 6.3 kg CO2eq
Biol Fertil Soils (2012) 48:981–985
ha−1 year−1 at a digestate application of 60 m3), which is a
minor impact (as discussed above). Therefore, despite the
small data set, this investigation clearly suggests that fugitive CH4 emissions after surface application of liquid digestate are negligible. If it is true that these emissions are
largely derived from dissolved gas (Wulf et al. 2002), i.e.
from a fixed (finite) amount, the magnitude of emissions
should be similar under different environmental conditions,
even though the temporal pattern of the efflux might vary.
Wulf et al. (2002) postulated that the extent of CH4
emissions in the field might be related to slurry dry matter
content. In their investigations, raw slurry with a higher dry
matter content than digestate infiltrated more slowly, maintained anaerobic conditions for longer and thus emitted
more CH4. This implies that digestate with a higher dry
matter content, e.g. deriving from dry AD processes, might
give rise to higher emissions of CH4 in the field. Moreover,
there are other reasons that aeration of the digestate might be
delayed, e.g. if it is injected into the soil (Wulf et al. 2002)
or used in paddy agriculture (Win et al. 2010). In these
cases, higher CH4 emissions than found by us might be
due to ongoing anaerobic degradation processes.
Acknowledgments This project was part of a larger study financed
by the Department of Agriculture, Fisheries and Food of the Republic
of Ireland, Research Stimulus Fund 07-506. We are grateful to Wolfgang Dieterich for helpful discussion. The technical assistance of
Brendan Burke and Padraig Brett at Oak Park is gratefully
acknowledged.
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