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.