Isotopic constraints confirm the significant role of microbial nitrogen oxides emissions from the land and ocean environment.

Nitrogen oxides (NO x , the sum of nitric oxide (NO) and N dioxide (NO 2 )) emissions and deposition have increased markedly over the past several decades, resulting in many adverse outcomes in both terrestrial and oceanic environments. However, because the microbial NO x emissions have been substantially underestimated on the land and unconstrained in the ocean, the global microbial NO x emissions and their importance relative to the known fossil-fuel NO x emissions remain unclear. Here we complied data on stable N isotopes of nitrate in atmospheric particulates over the land and ocean to ground-truth estimates of NO x emissions worldwide. By considering the N isotope effect of NO x transformations to particulate nitrate combined with dominant NO x emissions in the land (coal combustion, oil combustion, biomass burning and microbial N cycle) and ocean (oil combustion, microbial N cycle), we demonstrated that microbial NO x emissions account for 24 ± 4%, 58 ± 3% and 31 ± 12% in the land, ocean and global environment, respectively. Corresponding amounts of microbial NO x emissions in the land (13.6 ± 4.7 Tg N yr -1 ), ocean (8.8 ± 1.5 Tg N yr -1 ) and globe (22.5 ± 4.7 Tg N yr -1 ) are about 0.5, 1.4 and 0.6 times on average those of fossil-fuel NO x emissions in these sectors. Our findings provide empirical constraints on model predictions, revealing significant contributions of the microbial N cycle to regional NO x emissions into the atmospheric system, which is critical information for mitigating strategies, budgeting N deposition and evaluating the effects of atmospheric NO x loading on the world.