Abiotic nitrous oxide formation driven by sunlight in fresh and marine waters

Abstract

Nitrous oxide (N2O) is the main stratospheric ozone depleting agent, and the strongest greenhouse gas in the atmosphere. Classically, microbially-mediated processes, such as ammonia oxidation and denitrification are assumed to control the N2O budget in aquatic systems. Recent studies suggest that our understanding of N2O production is incomplete. Specifically, abiotic reactions like chemodenitrification may contribute significantly to N2O production, but their importance is uncertain, particularly in surface waters. Here, we experimentally demonstrate a significant and consistent production of N2O induced by sunlight under oxic abiotic conditions in two fresh and two marine waters, a novel process not considered for greenhouse gas budgets. N2O production was strongly correlated to the radiation dose. Nitrite is the main sustrate, and the nitrate can also contribute after being photoreduced to nitrite. We defined this abiotic N2O photoproduction as photochemodenitrification. Photochemodenitrification exceded biological production of N2O by ammonia oxidation in surface waters. We demonstrate that photochemodenitrification may be an essential and overlooked process occurring in fresh and marine surface waters across the globe, with particular importance in areas of high solar radiation and nitrite availability. Because it occurs in surface waters in direct contact with the atmosphere, photochemodenitrification may be responsible for a significant fraction of N2O emissions globally.