Transitional hypoxia during peatland water table fluctuations drives high N2O fluxes via shifting microbial pathways

Abstract

Nitrogen (N) cycling in peatlands is sensitive to variations in water table, temperature, and soil moisture. Water table fluctuations in peatlands can trigger the formation of transitional oxic and hypoxic zones within the peat and influence N2O fluxes. However, studies exploring these transitions by combining isotopic and microbial analyses are rare. This study assessed the impact of oxic-to-hypoxic transitions on N dynamics in drained peat soil. Birch-planted mesocosms were treated with 15N-NO3- and 15N-NH4+ tracers, while artificial fog generation allowed for the quantification of N2O exchange between the soil surface and the atmosphere. Rewetting reduced soil oxygen levels, significantly increasing N2O fluxes; peak value (162 ± 22.80 μg N m−2 h−1) was recorded under hypoxic conditions. During the transition, the abundance of nirK-type denitrifiers increased. N2O was initially enriched by the 15N-NH4+ tracer, followed by the 15N-NO3- tracer, which contributed significantly to the N2O flux under hypoxic conditions. The 3D FRAME isotope model attributed early N2O production to nitrification (Ni) and nitrifier denitrification (nD); however, it identified a dual contribution from nitrification (Ni) and bacterial denitrification (bD) during the later phases. NosZ genes were detected in birch leaf samples, suggesting a potential aboveground microbial N2O sink. 15N tracer also confirmed the solubility of soil N2O in fog droplets during the experimental period, but the fraction of dissolved N2O was found to be very low (<0.2%). Nonetheless, these findings warrant further research.