The role of abiotic processes in the formation and degradation of gaseous nitrogen compounds in the soil

Soils are a major source of nitrogen (N) trace gases, especially of nitrous oxide (N$_{2}$O) and nitricoxide (NO). The two microbial processes nitrification and denitrification are considered the major contributors to these emissions. While microbial denitrification has long been identified as a sourceof N trace gases under reducing conditions, N trace gas formation under aerobic conditions is far from being completely understood. Several abiotic reactions involving the nitrification intermediateshydroxylamine (NH$_{2}$OH) and nitrite (NO$^{-}_{2}$) have been identified leading to N$_{2}$O and NO emissions,but are neglected in most current studies. Further, there is a potential a biotic sink function of soils for N$_{2}$O via photochemical destruction. For better N trace gas mitigation strategies, the identification of the major source and sink processes and their role in the global N cycle is vital. Prior to the experimental work, this thesis reviews information about the role of abiotic processes in the formation of N trace gases from the few available studies reporting on a biotic emissions. It merges the gained information into a new conceptual model explaining the formation of the N trace gases N$_{2}$O, NO, as well as gaseous nitrous acid (HONO) by coupled biotic–abiotic reaction mechanisms. The relevant reactions are: the self-decomposition of NO$^{-}_{2}$, reactions of NO$^{-}_{2}$ with reduced metal cations, the nitrosation of soil organic matter (SOM) by NO$^{-}_{2}$, the comproportionation of NO$^{-}_{2}$ and NH$_{2}$OH, and the oxidation of NH$_{2}$OH by manganese or iron. While reactions involving NO$^{-}_{2}$ have been shown to produce primarily NO, reactions of NH$_{2}$OH are known to lead to N$_{2}$O as their main product. In soils it is difficult to discriminate between biological and abiotic processes. Here, stable isotope techniques are a promising tool to give more insight into the production processes. Especially the site preference (SP) of 15N in N$_{2}$O can help to source partition between processes. Experiments have been designed to study the abiotic formation of N$_{2}$O from NH$_{2}$OH in solutions and in different non-sterile and sterile soils from forest, grassland, and cropland. While organic forest soils showed hardly any N$_{2}$O formation upon NH$_{2}$OH addition, an immediate and strong formation of N$_{2}$O was observed in cropland soil, also in sterilized samples. A correlation analysis revealed apotential positive relationship of the NH$_{2}$OH-induced N$_{2}$O formation with soil pH and manganese content, construing an effect of pH on NH$_{2}$OH stability and of manganese acting as an oxidation agent for NH$_{2}$OH. A negative correlation between abiotic N$_{2}$O formation and C/N ratio was found that could indicate a possible competitive reaction of NH$_{2}$OH with functional groups of SOM. Allabiotic N$_{2}$O production pathways showed a characteristic, high SP unaffected by reaction conditions. [...