Microbial superoxide production drives 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 controlled factorial slurry experiments, native microbial communities produced approximately 10 times more NO₂ than sterile controls following nitric oxide (NO) exposure. Stimulating superoxide production with NADH increased NO₂ formation 15- to 26-fold, while inhibiting NADH oxidase reduced production toward baseline levels. Superoxide dismutase decreased NO₂ production by 46-71%, and O₂- 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 pathway linking carbon and nitrogen cycling where heterotrophic decomposers facilitate biogenic NO-to-NO₂ conversion via superoxide chemistry, potentially explaining discrepancies between satellite observations and modelled soil NO_x emissions.