Integrated isotope and microbiome analysis indicates dominance of denitrification in N₂O production after rewetting of drained fen peat

Peatlands are an important source of nitrous oxide (N₂O) emissions, which is a potent greenhouse gas and is also involved in the depletion of stratospheric ozone. Due to the large number of N₂O production and consumption processes, it is challenging to trace N₂O emissions to an individual process. We investigated the effect of different water regimes (dry, intermediate and flooded) on N₂O emissions via ¹⁵N tracing in a microcosm study with well-decomposed nitrogen-rich alluvial fen peat. The isotopic composition of the peat and emitted N₂O gas was analysed in combination with qPCR analysis of abundances and diversity of N-cycle functional genes. Bacterial denitrification was the predominant source of N₂O emission, followed by nitrification (ammonia oxidation). This was identified by a close relationship between ¹⁵N-N₂O and ¹⁵N-NO₃⁻ under flooded (anoxic) and intermediate (sub-oxic) treatments and concomitant increases in nirK, nirS and nosZ after the flooding. The site preference and δ¹⁸O values fell within the previously observed range indicating multiple overlapping processes and bacterial denitrification as the dominant process. Although the combination of isotopic and microbial analyses indicates that bacterial denitrification is the primary process under intermediate and flooded treatments, the high abundance of amoA indicates that nitrification via comammox was present. High archaeal amoA and bacterial amoA gene copy numbers and second highest N₂O emissions under the intermediate and flooded peats indicated that ammonia oxidation was, secondary to denitrification, also a source for N₂O. The increase in emissions with nrfA gene copies also showed that dissimilatory nitrate reduction to ammonium (DNRA) potentially contributed to N₂O emission under flooded treatment.