N2O dynamics in surface sediments of a seasonally euxinic coastal basin

The eutrophication and deoxygenation of coastal systems can severely impact the biogeochemistry of surface sediments, and can lead to oxygen limitation and sulfide accumulation. Changes in oxygen and sulfide availability may have large effects on the sedimentary dynamics of the potent greenhouse gas nitrous oxide (N₂O), and thus on N₂O fluxes between sediments and the overlying water column. Here, we used a combination of porewater and microsensor measurements, batch incubations, and metagenome and metatranscriptome analyses to assess the effect of oxygen and sulfide on N₂O production and consumption processes in surface sediments of a seasonally euxinic coastal system. In spring, our study system (Lake Grevelingen, The Netherlands) is characterized by oxygenated bottom waters and surface sediments, while water column stratification in summer leads to euxinic bottom waters and highly sulfidic surface sediments (mM concentrations). An absence of net N₂O production in spring sediment was consistent with an in situ limitation of oxygen and NO_x. Batch incubations showed that despite this in situ limitation, the microbial community maintained the potential for nitrification and N₂O production through denitrification. The nosZ gene, which is responsible for N₂O consumption, was present and expressed by a diverse microbial community dominated by clade II nosZ-possessing Flavobacteriia. Sulfidic summer conditions were simulated in batch incubations via sulfide additions. At low mM sulfide concentrations N₂O consumption was enhanced, while higher sulfide concentrations halted most of the studied nitrogen cycling processes. Hence, restoration of coastal systems by re-oxygenation could affect N₂O dynamics by changing oxygen, NO_x and sulfide availability, which would have implications for the role these sediments play in N₂O exchange with overlying waters.