Using 15N isotope and microbiome analysis to understand N2O production and consumption processes.

Nitrous oxide is a potent greenhouse gas which is involved in stratospheric ozone depletion. Even though nitrogen cycle has been studied for a long time, it is still challenging to understand specific N2O production and consumption processes. This is due to the complexity and heterogeneity of soil, wherein multiple processes can take place simultaneously. Isotopic composition of N2O can help us solve this and provide useful information on evaluating N2O sources and calculate global budgets. N2O is a linear molecule and its understanding at molecular scale can provide major insights into partition of its source processes. The N2O site preference (SP), which is the difference in 15N between N2O molecules substituted with 15N at the central and the peripheral position, has proved to be a major tool to tackle this problem. The objective of this study is to use isotopic and microbial research for N2O sources and process partitioning. We will bring some examples from our recent studies in the lab and in a drained peatland forest. During our lab study based on peat soil from a floodplain fen, we observed bacterial denitrification was a major source of N2O emissions under flooded conditions. We observed this using 15N isotopic mapping technique, which helped separate multiple active processes. We applied a similar method in-situ on a drained peatland in southeastern Estonia and described hybrid N2O formation, where one N atom of the N2O molecule was taken from NH4 and the other N molecule from another source such as organic N, was the dominant source of N2O emissions. The isotopic mapping and molecular enrichment of 15N during our experiment showed that. The isotopic mapping initially suggested nitrification as a major source, but on further investigation of 15N enrichment, we found the presence of hybrid processes (15N nitrogen from two pools or processes). Furthermore, we studied the genetic potential for major N2O processes (denitrification, nitrification, dissimilatory nitrate reduction to ammonium (DNRA)) and combined these with the isotope results, and this integrated approach is an important tool to partition N2O processes. When using 15N tracers, the isotopic technique can partition the sources (nitrate or ammonia) of N2O. Hence, using the isotopic mapping of natural abundances and 15N tracers to partition the source, we can get initial insights into N2O sources and processes together. Isotopic mapping is still under development and further research is required as it also has a problem of overlapping of processes.