Microbial superoxide production influences biogenic nitrogen dioxide formation in soils

Nitrogen dioxide (NO₂) is a critical atmospheric pollutant and ozone precursor, yet biogenic soil sources remain poorly constrained. Current models assume soil NO₂ flux is exclusively depositional. Here we demonstrate that soils can produce NO₂ through microbial superoxide (O₂^–) production. Using manipulative slurry experiments, native microbial communities produced 6-10 times more NO₂ than sterile controls following NO exposure. Stimulating superoxide production with NADH increased NO₂ formation 15-fold, while inhibiting NADH oxidase reduced production to near-sterile levels. Superoxide dismutase decreased NO₂ production by 50-75%, and superoxide concentration explained 60% of variation in NO₂ production rates. Addition of peroxynitrite to soil increased headspace NO₂, confirming this intermediate as the mechanistic link. These findings reveal a novel pathway linking carbon and nitrogen cycling where heterotrophic decomposers facilitate biogenic NO to NO₂ via superoxide chemistry, potentially explaining discrepancies between satellite observations and modelled soil NOx emissions.