Effekte unterschiedlicher Langzeitdüngerstrategien auf Humusgehalt und N2O Emissionen landwirtschaftlich genutzter Böden

Abstract

Agricultural soils represent a source of N2O and CO2 which are the major greenhouse gases and should be reduced to diminish global warming. The increase of organic matter in soils, which can be reached by the long-term application of organic fertilizer, is an important factor to improve soil quality and to bind atmospheric CO2. However, the influence of long-term organic fertilization on trace gas emissions is not well understood. Therefore, the aim of the present study was to compare the emissions of N2O and CO2 in different long-term field experiments with addition of organic and mineral or no fertilizer. The key question of this thesis is whether N2O emissions, which are produced during the microbial processes of nitrification and denitrification, increase due to higher organic C stocks. On the one hand higher C stocks can increase microbial activity which in turn depletes the oxygen in the soil. This leads to optimal conditions for denitrification. On the other hand C is used as a substrate for heterotrophic nitrification and denitrification. During this thesis the following four studies were performed to investigate the influence of organic fertilization on N2O emissions. The first experiment was a laboratory incubation with the sandy soil from a long-term fertilization experiment at Darmstadt. The effects of long-term fertilization (organic vs. mineral) on N2O emissions were determined at different soil moisture levels. Further, these long-term effects were compared with the short-term emissions following the application of different fertilizers (KNO3, farmyard manure and biogas waste). The second study was another incubation experiment, which was performed with soils from three different long-term experiments with similar fertilization history, but different texture and C stocks. The question underlying this study was whether different soil types lead to different C stocks and whether they have a different impact on N2O emissions. For this experiment, emission rates of N2O and CO2 were measured at a constant soil moisture content of 60% water-holding capacity, following the application of different fertilizers (KNO3 vs. farmyard manure from cattle) and after simulation of a heavy rainfall event, which increased soil moisture to field capacity. The third experiment was a two-year field study, which was conducted to determine the effect of increased organic C stocks on N2O emissions under field conditions. Gas fluxes were measured weekly with closed chambers on the sandy soil at the Darmstadt long-term fertilization experiment. In the fourth study a validation and calibration approach was tested to describe the field experiment at Darmstadt using the DNDC model. The results showed that increased C stocks only play a minor role in terms of N2O emissions. Two of the three soils investigated in the second study showed slightly increased N2O emissions when incubated at 60% water-holding capacity. However, the effect of fertilization history and increased organic carbon contents on N2O emissions was small. Moreover, these effects were not detectable anymore following the application of fertilizers, which resulted in large emissions independent of the fertilization history. On the sandy soil from the Darmstadt site, the higher organic C stocks did not affect N2O emissions, neither during the laboratory incubation (first study) nor during the field experiment (third study). This was attributed to the sandy soil texture and low soil moisture content during the incubation and during the field experiment. During the laboratory study, short-term emissions after fertilization were much more pronounced. Especially the application of biogas waste was followed by very high N2O emissions. Tillage combined with fertilization led to the highest emissions in the field. The second laboratory incubation showed much higher emissions from a soil which was depleted of organic matter due to the lack of fertilization for 25 years. This indicates that a sustainable soil humus management is necessary to regulate N2O emissions. Modelling C and N dynamics using the DNDC model indicated that the model was useful for a prediction of N2O emissions after site-specific cal! ibration when the same fertilizer type was used at a different rate. However, the model failed when a different fertilizer type was used. Summing up, organic C stocks can influence N2O emissions but only to a small extent and under certain conditions. On sandy soils, where soil moisture tends to be low, the risk of increased N2O emissions at higher organic C stocks is probably low. Further, short-term effects like fertilization events influence emission dynamics to a much higher extent. Finally, it appeared that a balanced nutrient and humus management can even avoid N2O emissions by it positive effects on soil structure. Our results indicate that feedback mechanisms of soil carbon sequestration on N2O emissions have to be considered when discussing options to increase soil carbon stocks.