Diazotrophs: An Overlooked Sink of N2O

Abstract

Abstract The ocean is the second‐largest source of greenhouse gas nitrous oxide (N2O). However, its role as an N2O sink is severely overlooked. N2O fixation by diazotrophs has lately been proposed as a new pathway of N2O consumption. We investigated diazotrophic N2O consumption and examined the anthropogenic influence on N2O dynamics in the coastal northeastern Arabian Sea, a hotspot of N2O emissions. Our findings reveal that relatively unperturbed waters, unlike anthropogenically perturbed waters, are a modest net N2O sink (98 ± 29% saturation), contrary to previous reports. N2O fixation remains active in anthropogenically perturbed waters in contrast to N2 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 N2O fixation not only directly sequesters N2O but may correspond to 0.3 Tg C y−1 of global ocean net primary production.

Keywords

• N2 fixation
• N2O fixation
• greenhouse gases
• primary production
• Arabian sea
• Nitrogen cycle