The relative dominance of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) under four vegetation types in a typical coastal wetland

Denitrification (DNF) and dissimilatory nitrate (NO3−) reduction to ammonium (DNRA) are the two most important biological processes during NO3− reductions in soil, which participating in the N2O production balance in ecosystem. However, the relative contribution and driving factors of these two biological processes in coastal wetlands, and their relationship with different vegetation/seasonal changes have not been well studied. We investigated the DNF, DNRA and the bacterial community structure with different vegetation belts (mudflat (no plant growth, Mud.), Spartina alterniflora (Spa.), Phragmites australis (Phr.), and the intergrowth area of Phragmites australis and Spartina alterniflora (Int.) in summer and winter at a coastal wetland in Chongming Dongtan, China in 2018. Results showed that the DNF rate was highest in the Phr. soil in summer, and in the Spa. soil in winter. In contrast, the DNRA rate was found highest in the Spa. soil than other belts in summer. Both the DNRA and DNF rates in the Spa. soil was higher in winter than those in summer. And the DNRA/DNF ratios ranged from 5 to 10 across different vegetation types and seasons. The DNRA, rather than DNF, was the dominating process in NO3− reduction in this coastal wetland. Interestingly, the DNRA/DNF ratios were positively correlated with soil NO3− concentrations. It was found that at phylum level Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the predominant bacteria. Clostridiales, Gammaproteobacteria and Ramlibacter were observed in summer but disappeared in winter; They were replaced by Bacillaceae, Lysinibacillus and Paenibacillus in winter. This study has important implications for facilitating N conservation in soils and alleviating N2O emissions in coastal wetlands through harvesting withered plants in winter, particularly for Spa. to reduce the input of organic C into soils and enhance DNRA rates substantially.