Estuarine and coastal environments are assumed to contribute to nitrous oxide (N2O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N2O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N2O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N2O molecule increased due to increasing nitrate concentration, suggesting the changes in N2O production pathways. Enhanced N2O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N2O production by 11-25%, whereas it decreased bacterial N2O production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of N2O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N2O production. Collectively, these results suggest that increasing nitrate concentration can shift N2O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N2O production and have widespread implications for N2O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.
Keywords: bacterial denitrification; estuarine sediment; fungal denitrification; nitrate; nitrous oxide.