15N tracers and microbial analyses reveal in situ N2O sources in contrasting water regimes on drained peatland forest

Managed peatlands are a significant source of nitrous oxide (N2O), a powerful greenhouse gas and stratospheric ozone depleter. Due to the complexity and diversity of governing microbial N2O processes, different methods such as tracer, isotopomer and microbiology are required to understand these processes. Their combined application helps to make precise estimates of the processes, which is crucial for the future management of drained peatlands, to mitigate soil degradation and negative atmospheric impact. In this study, we investigate N2O sources by combining tracer, isotopomer and microbial analysis in a drained peatland forest under flooded and drained treatments. On average, the nitrification genes showed higher abundances in the drained treatment, and the denitrification genes showed higher abundances under the flooded treatment. This is consistent with the underlying chemistry, as nitrification requires oxygen while denitrification is anaerobic. We observed significant differences in labelled N2O fluxes between the drained and flooded treatment sites. N2O emissions from the flooded treatment were nearly negligible, whereas the N2O evolved from the labelled 15NH4 in the drained site peaked at 147 μg 15N m−2 h−1. This initially suggested nitrification as the driving mechanism behind N2O fluxes in drained peatlands but based on the genetic data, isotopic analysis and N2O mass enrichment we conclude that hybrid N2O formation involving ammonia oxidation was the main source of N2O emissions under drained treatment. Based on the 15NO3 tracer addition and gene copy numbers, the low N2O emissions in the flooded sites came possibly from complete denitrification producing inert N2. On the atomic level, we observed selective enrichment of mass 45 of N2O molecule under 15NH4 amendment in drained treatment and enrichment of mass 45 and 46 both, under 15NO3 amendment in flooded treatment. The selective enrichment of mass 45 in drained treatment indicated towards presence of hybrid N2O formation, which was also supported by high abundances of archaeal genes.