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

Nitrogen oxides (NOx, the sum of nitric oxide (NO) and N dioxide (NO2)) 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 NOx emissions have been substantially underestimated on the land and unconstrained in the ocean, the global microbial NOx emissions and their importance relative to the known fossil-fuel NOx 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 NOx emissions worldwide. By considering the N isotope effect of NOx transformations to particulate nitrate combined with dominant NOx 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 NOx emissions account for 24 +/- 4%, 58 +/- 3% and 31 +/- 12% in the land, ocean and global environment, respectively. Corresponding amounts of microbial NOx 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 NOx emissions in these sectors. Our findings provide empirical constraints on model predictions, revealing significant contributions of the microbial N cycle to regional NOx emissions into the atmospheric system, which is critical information for mitigating strategies, budgeting N deposition and evaluating the effects of atmospheric NOx loading on the world. This study constructed new isotope mass-balance models, constrained fractions and emission amounts of major nitrogen oxides sources in land and ocean systems, and confirmed the significant contribution of microbial nitrogen cycles to global nitrogen oxides emissions.

Automated summary

Nitrogen oxide (NOx) emissions and deposition have significantly increased in terrestrial and oceanic environments, with microbial NOx emissions being underestimated on land and unconstrained in the ocean. This study provides empirical constraints on global NOx emissions, demonstrating the significant contribution of microbial nitrogen cycles to regional NOx emissions.