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

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.