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.