N2O reduction in soils

N2O is a known greenhouse gas that increased by 16% over the last 200 years. The main sources are native and agricultural soils where predominantly soil bacteria perform nitrification and denitrification with N2O as side and intermediate product, respectively. A complete denitrification, at which N2O is reduced to N2, is assumed to be the main elimination or sink process of N2O in the soils, beside dissolution in water. N2O sinks were first observed and measured in field studies. For a better understanding of the N2O sink processes laboratory studies with defined conditions were carried out, most of them under anaerobic conditions and airtight closed systems. We studied N2O sink processes in an experimental set up that had a continuous gas flow through soil samples to avoid a limitation of substrate gas concentrations over the experiment time. This set up allowed us to keep temperature and gas concentrations of N2O and O2 constant or selective to change them at discretion during the experiments. The concentrations of N2O and CO2 at the inlet and at the outlet of the incubation vessels could be measured continuously with a gas chromatograph. Therefore, we could determine the N2O consumption and the CO2 emission throughout the experiments. Our overall objective was to determine the ability and capacity of different soils to consume N2O at low oxygen concentrations and to determine the influence of certain parameters on N2O consumption and its rate. A change in the ratio of 15N to 14N as well as in the ratio of 18O to 16O in the N2O molecule was observed with coexistent raise of the N2O concentration in the atmosphere. This observation gives reason to believe that one of the N2O production or consumption processes might be the reason for the change in the fractionation factor. Our first objective was to check to what extend the N2O consumption could influence the isotopic signature in the remaining N2O molecules. Therefore, we determined the N2O reduction rate, the reduction rate constant and synchronously the isotopic signature of N and O in the remaining N2O that left the sample. We observed that with a decreasing reaction rate constant the fractionation factors for N and O increased and vice versa. We could as well determine the ratio of the stable isotopes N / O that lay mostly betweenandwith an average around 2.4, which is in agreement with other observations. We could conclude that the fractionation factors of the stable isotopes N and O depend on the N2O reduction rate constant and that the ratio of the enrichment factors for the stable isotopes N and O is constant. This ratio can help to detect N2O consumption if reduction is the dominating process in the turnover of N2O. The predominant faith of N2O in soils is either the emission to the atmosphere, the dissimilatory reduction to N2 or the dissolution in water. However, other types of N2O sink were observed, for example N2O fixation with following transforming to NH3. Hence, our second objective was to test, if these observations and a possible incorporation of N-N2O into soil organic matter are of ecological relevance in soils. We approached this problem by using labelled 15N2O as the only nitrogen source for four different soils over several days in our flow through set-up. We measured N2O consumption continuously and δ15N of soil organic matter before and after the experiment. The results revealed an average of the 15N enrichment in the soil organic matter of about 0.019%. Therefore, we could conclude that assimilatory reduction of N2O plays a negligible role as a N2O sink in soils, at least for our tested soils.
Although denitrification and therefore N2O consumption is known to occur under anaerobic conditions there are observations of N2O consumption at low O2 or aerobic conditions. This could indicate that N2O consumption is a more widespread and important process in soil as assumed. Our third objective of this thesis was to study the ecological relevance of N2O consumption in view of the total respiration in soils. Thereby, N2O and total respiration rates and the Michaelis-Menten Constant (km) values for N2O consumption were determined at different temperatures and oxygen concentrations. The received km values were between 1.8 – 10.4 ppm in soil gas phase (0.045 – 0.26 μM in soil solution) at all applied temperature and oxygen concentrations. This range fits with km values of other observations and suggests that there might be a common range of km values for N2O consumption in soils. In contrast, the km values determined for pure cultures of N2O reducing bacteria were about 9 times larger (2.4 – 7.5 μM in soil solution, converted: 96 -300 ppm). This result might point to a higher affinity of the N2O reducing enzyme in soils than in pure cultures. The ratio of N2O to total respiration gave us the amount of electrons that were transmitted to a N2O molecule instead to O2. This was for our soils up to 1.25%. Our observations showed that N2O and aerobic respiration could occur simultaneously probably in different microhabitats within the soil.
Only 1% of the microbes in soils have the ability to produce the enzyme N2O reductase that reduces N2O to N2. The production of the enzyme is controlled by environmental influences like oxygen concentration and the concentration of the denitrification intermediate products. Our objective for this part was to find out to what extent N2O treatments could influence the microbial community and the N2O reducing bacteria. We used two different DNA fingerprinting methods, RISA (ribosomal RNA intergenic spacer region analysis) and DGGE (denaturation gradient gel electrophorese) on four different soils, which showed N2O consumption. Through this method, we could conclude that all tested soils have strong differences in their microbial community. The treatment of the soils caused a shift in the microbial community, but it was not clear which of the parameter the high temperature, low oxygen, and/or high N2O concentration, had the most effect on the microbial community.
We could prove that all soils we tested have the ability to reduce N2O to N2 at low oxygen concentrations. The potential of this process depends highly on the N2O and O2 concentrations, temperature, and aggregate sizes in the soil. Altogether, we enhanced our knowledge about the N2O consumption process and could conclude that this process is of ecological importance in soils.