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

Coastal eutrophication and deoxygenation can lead to sulfide accumulation, which can severely impact the dynamics and fluxes of the potent greenhouse gas nitrous oxide (N2O). Here, we investigated microbial N2O dynamics in surface sediments of a seasonally stratified coastal system, of which the water column is oxygenated in spring, whereas in summer its bottom waters are euxinic with highly sulfidic surface sediments. During oxygenated spring conditions, rapid depletion of the substrates oxygen (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 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 indicate that coastal sediments can maintain significant N2O turnover potential despite substrate limitations and elevated sulfide. Consequently, ecosystem restoration strategies that alter O2, NOx and sulfide availability may fundamentally impact coastal N2O budgets.