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

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.