Unlocking soil nitrous oxide sink through labile carbon.

Nitrous oxide (N2O), a potent greenhouse gas and the dominant ozone-depleting substance, is primarily emitted from soils. Understanding atmosphere-to-soil N2O uptake is crucial for harnessing the soil's natural sink potential, yet its quantification and mechanistic drivers remain poorly constrained. Here, we disentangled gross N2O emission and uptake using the 15N2O pool dilution method, revealing key drivers in five cropland soils across China and in labile carbon-amended soils. In particular, the ratio of gross N2O uptake to emission-driven by the soil labile carbon to inorganic nitrogen [dissolved organic carbon/dissolved inorganic nitrogen (DOC/DIN)] ratio-showed pronounced variability (0.28 to 0.79), revealing a powerful but previously underestimated lever for regulating net N2O fluxes. Mechanistically, an elevated DOC/DIN ratio promoted complete denitrification by increasing electron supply, stimulating oxygen-depleting respiration, and enhancing nosZ gene abundance, achieving up to 51% N2O mitigation. Our findings underscore soils as dynamic N2O regulators and propose a carbon-nitrogen-coupled regulation framework, where labile carbon plays a critical role in regulating N2O dynamics.