Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments
Xiaofei Li, Dengzhou Gao, Ye Li, Yanling Zheng, Hongpo Dong, Xia Liang, Min Liu, Lijun Hou
Abstract
Estuarine and coastal environments are assumed to contribute to nitrous oxide (N 2 O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N 2 O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N 2 O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N 2 O molecule increased due to increasing nitrate concentration, suggesting the changes in N 2 O production pathways. Enhanced N 2 O 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 N 2 O production by 11–25%, whereas it decreased bacterial N 2 O production by 16–33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13–28% of N 2 O production. Elevated nitrate concentration significantly mediated nirK -type denitrifiers structure and abundance, which are the keystone taxa driving N 2 O production. Collectively, these results suggest that increasing nitrate concentration can shift N 2 O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N 2 O production and have widespread implications for N 2 O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.