Transitional hypoxia during peatland rewetting drives high N2O fluxes via coupled microbial pathways with evidence of N2O sink potential by fog and tree leaves.

Peatlands are globally significant sinks for carbon and nitrogen. The carbon and nitrogen cycles in peatlands are highly sensitive to changes in water table, temperature, and soil moisture. Long-term rewetting has been suggested as a potential restoration strategy for peatland restoration; however, if done improperly, it can create transitional oxic and hypoxic zones, which can act as major hotspots for N₂O emissions. In this study, we investigated the effect of transitioning from oxic to hypoxic conditions in a drained peat soil prepared in mesocosms, which were installed in a climate chamber to simulate day and night conditions. The humidity in the climate chamber was maintained above 90% to study the interaction of the produced N₂O with artificial fog, generated using foggers. We prepared 12 mesocosms, out of which 4 received a ¹⁵N-NO₃^- tracer, 4 received a ¹⁵N-NH₄⁺ tracer, and the remaining 4 were kept as controls. One birch plant sapling was also planted in each mesocosm before the start of the experiment. Soil oxygen levels were reduced from 9 mg/L to 1.5 mg/L over the course of ten days, and the effects of this change from oxic to hypoxic conditions were studied.

Our results indicate that due to a decrease in soil oxygen over time, N₂O emissions increased and peaked on the final day (162 ± 22.80 μg N m^-2 h^-1) of the experiment. During this transition (oxic to hypoxic), we observed a significant increase in the abundance of nirK-type denitrifiers. Our ¹⁵N tracers indicate that on the initial days, the produced N₂O was dominated by ^{the 15}N-NH₄⁺ tracer, but on the final days, the ¹⁵N-NO₃^- showed a significant contribution to the N₂O flux. The birch sapling showed a major uptake of ¹⁵N-NO₃^- in its roots and leaves. This indicates a preference for birch saplings towards the soil nitrate pool compared to the soil ammonium. We also applied the 3D Frame isotope model to the natural isotopomers of soil-produced N₂O and observed a change in the N₂O production processes over the course of the experiment. The initial days were dominated by nitrifiers’ denitrification and nitrification; however, by the end of the experiment, isotopic mapping revealed the dominance of nitrification, coupled with bacterial and nitrifier denitrification. We also found evidence of the solubility of tracer-produced N₂O in the fog water.

Our study demonstrated that the improper restoration of peatlands through rewetting can create transitional oxic-hypoxic zones, which can serve as hotspots for N₂O emissions. Moreover, soil-produced N₂O can be dissolved in fog during colder seasons, which can be further coupled by tree leaves as they also possess potential for N-cycle processes.