Labile carbon supply modulates H₂O₂-mediated N₂O emissions during sediment denitrification: Insights from...

Reactive oxygen species (ROS), particularly hydrogen peroxide (H₂O₂), are critical yet complex regulators of the nitrogen cycle. While H₂O₂ is known to modulate nitrous oxide (N₂O) emissions during heterotrophic denitrification, how this regulation interacts with labile carbon supplies remains poorly understood. Here, we investigated the response of N₂O emissions to exogenous H₂O₂ gradients under varying carbon-to-nitrogen (C/N) ratios in riverine sediments. We found that labile carbon addition (glucose) significantly broadened the tolerance window of denitrification to H₂O₂ stress and altered the dose-response relationship of N₂O emissions across H₂O₂ concentrations ranging from 49 to 1960 μmol kg^-1 dry soil. Under carbon-limited conditions (NC), H₂O₂ reduced cumulative N₂O emissions by 36.98% during the initial 6 h, coinciding with decreased relative genomic representation of Class I complete-repertoire genera and Class II nosZ-bearing genera lacking at least one upstream module. Conversely, under high-carbon conditions, H₂O₂ addition resulted in a 20.74% increase in cumulative N₂O emissions compared to the control. Metagenomic analysis revealed a concurrent enrichment of denitrification and antioxidant genes (e.g., katG, trxB). This enriched genetic potential, contrasted with the observed N₂O accumulation, highlights an apparent uncoupling between genomic capacity and phenotypic activity. This suggests that while the microbial community retains the genetic potential for denitrification, acute oxidative stress likely constrains terminal N₂O reduction. These findings indicate that the convergence of labile carbon supply and ROS generation represents an important trigger for transient N₂O pulses. This study deepens the understanding of the role of H₂O₂ in regulating denitrification-derived N₂O emissions.