Autor:innen:
Dr. Pauline Sophie Rummel | Georg-August-Universität Göttingen | Germany
Paulina Englert | Georg-August-Universität Göttingen | Germany
Dr. Lukas Beule | Julius Kühn-Institut (JKI) Bundesforschungsinstitut für Kulturpflanzen | Germany
Victoria Nasser | Georg-August Universität Göttingen | Germany
Prof. Dr. Johanna Pausch | Universität Bayreuth | Germany
Prof. Dr. Klaus Dittert | Georg-August Universität Göttingen | Germany
Returning of crop residues is common agricultural management to prevent nutrient losses and increase soil fertility. However, acceleration of N- and C-cycling processes often lead to increased losses of greenhouse gases. Litter and soil organic matter (SOM) turnover increase microbial respiration, and thus O2 demand. This may lead to formation of local hotspots with anoxic or microoxic conditions providing favorable conditions for denitrifiers.
As denitrification is controlled by many interacting factors, disentangling the effect of a single factor under field conditions is challenging. Thus, we conducted a series of laboratory incubation experiments under controlled conditions with variation of either soil type, soil moisture, litter type, or N availability. We compared soils with different texture and SOM content (arable and grassland), soil moisture (50-70 % WFPS), and litter quality (C:N ratio 9-82). CO2, NO, N2O, and N2 were measured continuously and we applied the N2O isotopocule mapping approach to distinguish between N2O emitting processes. Soil water extractable organic C and inorganic N were measured in parallel samples. At the end of the experiment, microbial biomass, abundance of bacteria, fungi, and N-cycling genes were assessed.
Litter addition significantly increased CO2, N2O, and N2 losses irrespective of litter quality, soil moisture or soil type/SOM content. Under O2-limiting conditions, bacterial denitrification was the main source of gaseous N losses. CO2 and N2O or N2O+N2 fluxes were strongly positively correlated confirming that easily degradable C promoted denitrification. This relationship was not true for wheat straw which led to high CO2 losses but low N2O+N2 losses. N addition decreased CO2 losses but increased N2O and N2 losses in all litter treatments. The effect decreased with increasing litter C:N but changes in the N2O/(N2O+N2) ratio were not consistent among litter types.
Under moderate soil moisture, interactions between litter degradation and SOM turnover affected the time course and processes contributing to NO and N2O formation. The NO/N2O ratio indicated contribution of both nitrification and denitrification in the grassland soil. Isotopocule mapping confirmed contribution of different processes depending on soil type, moisture, and litter addition. In the grassland soil, fungal denitrification contributed to N2O formation after litter addition, while nitrification contributed to N2O formation without litter addition.