Diazotrophs: An Overlooked Sink of N2O

Abstract

The ocean is the second-largest source of greenhouse gas nitrous oxide ($N_2$O). However, its role as an $N_2$O sink is severely overlooked. $N_2$O fixation by diazotrophs has lately been proposed as a new pathway of $N_2$O consumption. We investigated diazotrophic $N_2$O consumption and examined the anthropogenic influence on $N_2$O dynamics in the coastal northeastern Arabian Sea, a hotspot of $N_2$O emissions. Our findings reveal that relatively unperturbed waters, unlike anthropogenically perturbed waters, are a modest net $N_2$O sink (98 $\pm$ 29$\%$ saturation), contrary to previous reports. $N_2$O fixation remains active in anthropogenically perturbed waters in contrast to $N_2$ fixation. We additionally provide evidence that the absence of control incubations leads to incorrect fixation rate estimates, further implying that oceanic dark carbon fixation rates might be overestimated. We suggest that $N_2$O fixation not only directly sequesters $N_2$O but may correspond to 0.3 Tg C $y^{-1}$ of global ocean net primary production.