Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments.

Estuarine and coastal environments are assumed to contribute to nitrous oxide (NO) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying NO production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate NO production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the NO molecule increased due to increasing nitrate concentration, suggesting the changes in NO production pathways. Enhanced NO 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 NO production by 11-25%, whereas it decreased bacterial NO production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of NO production. Elevated nitrate concentration significantly mediated -type denitrifiers structure and abundance, which are the keystone taxa driving NO production. Collectively, these results suggest that increasing nitrate concentration can shift NO production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced NO production and have widespread implications for NO projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.