Biogas production has increased rapidly in Germany, resulting in large amounts of biogas
slurries. Physico-chemical properties of biogas slurries differ from those of conventional
animal slurries. Due to this fact a field study has been established to determine ammonia
losses by volatilization after supply of biogas slurries in grassland. Perennial ryegrass (
Lolium
perenne
) was tested as an alternative to conventional biogas crops (silage maize, silage
cereals) in the marsh region of Schleswig-Holstein in Northern Germany. Yield was
determined under different N-fertilisation-levels of mineral fertiliser (CAN) and biogas slurry
applied by trail hoses. Ammonia losses are mainly influenced by climate conditions and were
investigated by a micrometeorological approach, backwards Lagrangian stochastic dispersion
method. Because of strong wind in the coastal region the major fraction of the ammonia
emissions were observed within the first 10 hours after application. The NH
3
-losses by
volatilization were higher than in other regions of Schleswig-Holstein and substantially
decreased yield of perennial ryegrass.
Keywords: Ammonia volatilization, Biogas slurry, Fertiliser value, Energy crop rotation
Introduction
Biogas plays an important role in the German bio-energy production, with energy crops
(silage maize, silage cereals) particularly grown as fermentation substrates. It is not only in
coastal regions of Germany that grass can also serve as an alternative to the predominant
biogas-crop, maize. However, the overall environmental benefit from growing of energy
crops may be reduced by NH
3
-emissions after application of biogas slurries, unavoidable in
these production systems. NH
3
-emissions are among the main sources of acidifying and
eutriphying atmospheric compounds and are considered as indirect greenhouse gas emissions.
Furthermore the N-fertilisation value of organic fertilisers can strongly be reduced by NH
3
-
volatilization (Sommer and Hutchings, 2001). Marshlands are typical regions for grass
cultivation and characterised by very particular growing conditions because of shallow
groundwater levels, heavy, silt-clayey soils (Fluvimollic Gleysol) and strong wind speeds
which can also strongly influence NH
3
loss processes. There exists only very limited
information on the potential NH
3
-losses under such conditions.
Materials and methods
Ammonia (NH
3
) losses after application of biogas slurries on grassland by trail hoses were
determined on larger field plots (1 plot per fertilisation) by using the micrometeorological
backwards Lagrangian stochastic dispersion technique (bLS, Sommer
et al.
, 2005) in the
marsh of the federal state of Schleswig-Holstein, Northern Germany. The plot size of each
fertilisation date depended on the working width of the trail hoses, which varied between the
different dates of fertilisation (plot sizes at investigated fertilisations: 07.04.09: 48 m x 48 m;
Grassland Science in Europe, Vol. 15
268
28.05.09: 54 m x 54 m; 01.07.09: 36 m x 36 m). Yield of perennial ryegrass was investigated
in a multi-plot field trial including a bi-annual energy crop rotation consisting of maize (
Zea
mays
), wheat (
Triticum aestivum
), Italian ryegrass (
Lolium multiflorum
) and the monocultures
of maize and perennial ryegrass. The latter was cut four times. Nitrogen fertilisers tested in
this study included co-fermented biogas slurry (BGS) and CAN as mineral fertilizer. Besides
the control treatments, N was applied in two levels (360 kg ha
-1
subdivided in 120/120/70/50
and 480 kg ha
-1
subdivided in 165/145/100/70 kg ha
-1
). Plot size was 12 m x 12 m with four
replicates for each treatment. Ammonia loss measurements after biogas slurry application
were carried out on the same date and with the same N-supply as for the moderate N-level
treatment (360 kg ha
-1
N). Ammonia losses from CAN were considered as negligible. Yield of
perennial ryegrass [t ha
-1
] was statistically analysed by two-way ANOVA including N-form
and N-levels as treatment factors.
Results and discussion
Nitrogen losses by NH
3
-volatilization differed between dates of fertiliser application. Figure 1
shows the cumulative and the relative ammonia losses after biogas slurry application on
grassland determined in 3 experimental campaigns.
Figure 1 Cumulative and relative N-loss by NH
3
-volatilization after application of biogas
slurry in different N levels, perennial ryegrass (
Lolium perenne
), marsh region of Schleswig-
Holstein, micrometeorological bLS-technique, 3 dates during spring and summer 2009.
After N-application relative losses of about 20% of NH
4
-N applied could be observed on the
application dates 7 April and 1 July, whereas on 28 May the NH
3
-losses were higher than
30% of supplied NH
4
-N. The higher relative losses were probably due to strong wind on 28
May, which is one of the main driving factors for NH
3
-volatilization. Average wind speed at a
height of 2 metres for the first few hours after application was 7.4 m s
-1
, much higher than on
7 April (3.6 m s
-1
) and on 1 July (2.4 m s
-1
). NH
3
-volatilization losses after slurry application
Grassland in a Chan
g
in
g
World
269
with trail hoses in the marsh were about two fold higher than in other landscapes of
Schleswig-Holstein
(Pacholski
et al.
, 2009).
The yield of ryegrass was significantly lower for every cut in the biogas slurry treatment as
compared to mineral fertilisation. No interaction between N fertiliser and N level was
observed (Fig. 2).
Figure 2 Dry matter yield [t ha
-1
] of perennial ryegrass, cut 4 times a year, at a moderate (360
kg ha
-1
N) and a high level (480 kg ha
-1
) of N fertilization with biogas slurry (BGS) and
mineral fertilizer (CAN) in the marsh region of Schleswig-Holstein. Values with different
letters are statistically different at
P
<0.05 (Tukey-test, n=4).
As a result the total DM yield decreased from 14.5 t ha
-1
(CAN) to 8.5 t ha
-1
(BGS) by about
42% under the lower N level, and from 17.3 t ha
-1
(CAN) to 10.4 t ha
-1
(BGS) about 40%
under the high N level (Fig. 2). The data suggest that N-losses by NH
3
-volatilization were the
main factor for high differences in yield levels between the two N-fertilisers. In addition,
lower yields under biogas slurry fertilisation could also be due to ammonium adsorption and
fixation in the soil characterized by high clay contents, N
2
O-losses by microorganisms or N-
leaching. Besides environmental impact, like acidification and eutrophication, NH
3
-
volatilization can thus also strongly influence the N-fertilisation value of biogas slurry.
Nevertheless, a DM yield of 14.5 t ha
-1
for grass with mineral fertilisation was competitive to
the yield of dominant biogas crop maize at this site (average of 15 t ha
-1
).
Conclusion
Fertilisation of biogas slurries with trail hoses can lead to high N-losses by NH
3
-volatilization.
Losses were even higher than 30% of NH
4
-N applied, in situations with strong winds. Those
losses substantially contributed to strongly decreased yields of perennial ryegrass and high
environmental impact. In the study region, grasses cut 4 times a year can be considered as a
realistic alternative to maize as substrate for biogas production due to similar dry matter yield
levels.
References
Pacholski A., Gericke D. and Kage H. (2009) Measurement and modelling of NH3 emissions from field-applied
biogas residues in North German energy crop rotations. In: Grignani C., Acutis M., Zavattaro L., Bechini L.
Bertora C., Marino Gallina P. and Sacco D. (eds.)
Proceedings of the 16th Nitrogen Workshop - Connecting
different scales of nitrogen use in agriculture
. 28th June – 1st July 2009, Turin, Facolta di Agraria, Turin, Italy,
189-190.
Sommer S.G. and Hutchings N.J. (2001) Ammonia emission from field manure and its reduction – invited paper,
European Journal of Agronomy
15, 1-15.
Sommer S.G., McGinn S.M. and Flesch T.K. (2005) Simple use of the backwards Langrangian stochastic
dispersion technique for measuring ammonia emission from small field-plots.
European Journal of Agronomy
23, 1-7.