Missing flux from the ocean: isotope revealed an important NOx flux mediated by microbial processes.

NOx in the atmosphere can affect human health and economic development as well as atmospheric chemistry and climate. Thus, it is crucial to deeply understand the dynamics of NOx including its sources and sinks. However, as in the cases of other nitrogen compounds such as nitrous oxide, it is quite difficult to trace the fate of NOx, then close its budget.

Song et al. [1] nicely tackled this issue with the help of the 15N signature of NOx. They summarized the δ15N data of NO3− in atmospheric particulates observed in the ocean sites, which are quite low. Even after the correction of the potential changes in δ15N, the low δ15N signatures of ocean NOx (−12.5‰ on average) cannot be explained by δ15N of NOx from the oil combustion of marine traffic transportation (+4.7‰ on average), which has been considered as a single NOx emission source in marine environments. The simple isotope mass-balance model required them to include one more NOx emission source with low δ15N: microbially derived NOx.

The contribution of microbial NOx emission was estimated as 8.8 ± 1.5 Tg-N/yr [1]. Compared to the total ocean NOx emission (15.2 ± 1.5 Tg-N/yr), this flux estimated by Song et al. [1] is shockingly high. However, this estimation looks reasonable when compared with other important nitrogen fluxes in the ocean such as denitrification (100–450 Tg-N/yr [2]), nitrous oxide emission (3.4 Tg-N/yr [3]) and nitrification (100–200 Tg-N/yr [4]). Many microbial processes such as nitrification and denitrification are known to produce NO, the precursor to atmospheric NOx. Song et al. [1] appealed the urgent necessity for more research on the microbial NOx. It should be noted that although the δ15N signature of NOx is quite powerful in elucidating the missing NOx fluxes as found in Song et al. [1], the estimation, inevitably, heavily depends on the endmembers δ15N data of NOx and isotopic changes during the formation of particle NO3−, both of which are quite difficult to obtain and to constrain. More intensive research on the production/consumption of NOx as well as isotopic fractionations during these processes can make great contributions to constraining the microbially derived NOx fluxes in the ocean and land that are now beautifully unlocked by Song et al. [1].