Nitrous Oxide Production in Agricultural Soil – Linking Biogeochemical Pathways and Drivers

Abstract

Nitrous oxide (N2O) is a long-lasting and potent greenhouse gas responsible for depletion of stratospheric ozone. As the atmospheric N2O concentration reaches all-time highs, emission variability in space and time still leaves unresolved questions. The aim of this thesis is to improve our understanding of the origin of N2O and its main drivers from the largest anthropogenic source: agricultural soil. Therefore, we investigated agricultural soil from long-term trial field sites in the laboratory and used 15N-enriched tracers in two main approaches: partitioning of the sources of N2O production and quantification of the gross rates of microbial processes competing for ammonium (NH4+) and nitrate (NO3-). The varying relative contribution of NH4+, NO3- and organic nitrogen (Norg) to N2O emission highlights the influence of site-specific factors apart from the field management. Without fertilizer, Norg was the dominant N2O source related to high carbon (C) contents and C:N ratios. High N2O emissions were caused by increasing contributions of nitrification and denitrification, which was drastically stimulated by mineral nitrogen (N) fertilizer. In addition, N fertilizer application more than doubled N2O production from native non-fertilizer N compounds, which provides evidence for primed N2O production. By using the Ntrace model, we quantified gross rates of N cycle processes that compete for substrates and regulate N2O production. In the long term, cropping systems can shift the balance between denitrification and dissimilatory nitrate reduction to ammonium (DNRA), which determines the fate of NO3- in soil. A perennial cropping system that maintains high SOM contents and C/NO3- ratios has shaped the microbial community of dissimilatory nitrate reducers leading to higher N retention by DNRA and lower N2O emissions. By applying selective inhibitors, we were able to quantify the specific activity of archaeal and bacterial nitrifiers competing for NH4+. While both can coexist and be equally active in agricultural soil with low N supply, bacteria outcompeted archaea with increasing NH4+ concentration, which can be responsible for higher N2O emissions as well. This thesis illustrates how human action drives N2O emission from agricultural soil in a variety of ways since field management affects N cycle processes in the short- and long-term. While N fertilizer application strongly stimulates N2O production from added- and native N sources, long-term field management can change the soil properties, which shifts the abundance of microbial communities and thereby alters the N cycle processes responsible for N2O production.