Coastal sediments maintain potential for N2O cycling under seasonally varying redox conditions

Coastal eutrophication can lead to deoxygenation and sulfide accumulation in sediments, which could strongly impact the dynamics of the potent greenhouse gas nitrous oxide (N2O). Here, we investigated the effects of oxygen (O2) and sulfide on microbial N2O production and consumption in surface sediments of a seasonally euxinic (anoxic and sulfidic) coastal basin. In spring, these surface sediments are oxygenated, while they are highly sulfidic during stratification of the water column in summer. During oxygenated spring conditions, rapid depletion of the substrates O2 and nitrate (NO3-) in the surface sediment limited net in situ N2O production, despite potential for nitrification and for N2O production through incomplete denitrification as observed in batch incubations. Based on metagenome and metatranscriptome analyses the N2O-consuming microbial community was shown to be highly diverse and dominated by clade II nosZ-possessing Flavobacteriia. Assessing the summer sulfidic conditions in surface sediments via batch incubations, we found that moderate sulfide concentrations (0.2-1 mM) enhanced N2O consumption, whereas high concentrations (4 mM) inhibited all steps of denitrification. These findings highlight the redox controls on N2O dynamics, by indicating that coastal sediments can maintain significant N2O turnover potential despite substrate limitations and elevated sulfide concentrations. Consequently, ecosystem restoration strategies that alter O2, NO3- and sulfide availability may fundamentally impact coastal N2O budgets.