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 δ¹⁵N 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.