Microbial superoxide production drives biogenic nitrogen dioxide formation in soils

Abstract Nitrogen dioxide (NO 2 ) is a critical atmospheric pollutant and ozone precursor, yet biogenic soil sources remain poorly constrained. Current models assume soil NO 2 flux is exclusively depositional. Here we demonstrate that soils can produce NO 2 through microbial superoxide (O 2 ˉ) production. Using controlled factorial slurry experiments, native microbial communities produced approximately 10 times more NO 2 than sterile controls following nitric oxide (NO) exposure. Stimulating superoxide production with NADH increased NO 2 formation 15- to 26-fold, while inhibiting NADH oxidase reduced production toward baseline levels. Superoxide dismutase decreased NO 2 production by 46-71%, and O 2 ˉ concentration explained 60% of variation in NO 2 production rates. Addition of peroxynitrite to soil increased headspace NO 2 , confirming this intermediate as the mechanistic link. These findings reveal a pathway linking carbon and nitrogen cycling where heterotrophic decomposers facilitate biogenic NO-to-NO 2 conversion via superoxide chemistry, potentially explaining discrepancies between satellite observations and modelled soil NO x emissions.