| Gawan Heintze1,2, Matthias Drösler1, Ulrike Hagemann3and Jürgen Augustin3
1University of Applied Sciences Weihenstephan-Triesdorf, Chair of Vegetation Ecology,
Weihenstephaner Berg 4, 85354 Freising, Germany
2Technische Universität München, Chair of Plant Nutrition, Emil-Ramann-Str. 2, 85354
Freising, Germany
3Leibniz Centre for Agricultural Landscape Research (ZALF), Eberswalder Straße 84,
15374 Müncheberg, Germany
Together with industrial process-related emissions (8.1%) the actual GHG emissions from
agriculture (7.5% - 70 million tones (Mt) of carbon dioxide (CO2)-equivalents) representing
after energy-related emissions from combustion processes of fossil fuels (83.7%) the
second largest budget of the Germany-wide total emissions per year. To reduce the
EU’s CO2 emissions by 20% by 2020 the cultivation of energy crops for biogas
production, ideally coupled to a subsequent return of the resulting residues in form of
biogas digestate is intended as one key element in the pathway of renewable energy
production. Despite an increasing cultivation of energy crops for the production of
biogas aiming to reduce the overall climate impact of the agricultural sector, it is still
largely unknown how the application of ammonia-rich organic digestate effects
field N2O emissions. Therefore, the collaborative research project "potential for
reducing the release of climate-relevant trace gases in the cultivation of energy
crops for the production of biogas" was launched. The main objective of the study
was to determine an improved process understanding and to quantify the influence
of mineral nitrogen fertilization, biogas digestate application, crop type and crop
rotation, to gain precise and generalizable statements on the exchange of trace gases
like nitrous oxide (N2O) and methane (CH4) on the resulting climate impact. Gas
fluxes of N2O and CH4 were measured for three and a half years on two differently
managed sites in maize monoculture with different applied organic N amounts and
in a crop rotation system called FFA and FFB with same amounts of applied N
but three different forms of N application (mineral N, mineral+organic N, organic
N).
The annual cumulative N2O exchange rates in maize monoculture showed a clear
dependence on the amount of applied organic fertilizer. Average annual cumulative exchange
rates ranged from 1.65 ± 0.74 kg N ha−1 yr−1 to 11.03 ± 1.63 kg N ha−1 y−1explainable by
a twice as high amount of N compared to the conventional fertilized site. The average annual
cumulative CH4 exchange rates in maize monoculture varied between -1.2 ± 0.46 kg C ha−1
yr−1 and 3.75 ± 0.48 kg C ha−1 y−1with measured CH4 fluxes around zero between the
fertilizing events, indicating a minor role. For FFA and FFB the average annual cumulative
N2O exchange rates ranged from 1.45 ± 0.18 kg N ha−1 yr−1 to 3.5 ± 1.1 kg N ha−1
y−1and 1.37 ± 0.57 kg N ha−1 yr−1 to 1.71 ± 0.29 kg N ha−1 y−1 andshowed lower values
to comparable treatments in the maize monoculture especially indicating the different
management effects. Determined average annual cumulative CH4 exchange rates
ranged from 0.19 ± 0.6 kg C ha−1 yr−1 to 0.21 ± 0.45 kg C ha−1 yr−1and -0.8 ±
0.7 kg C ha−1 y−1 to 1 ± 0.6 kg C ha−1 y−1 and played as well a minor role.
Altogether, biogas digestate can be seen as a suitable alternative if the amounts of
applied N selected appropriately in combination with a customized management. |