Hotspot of dissimilatory nitrate reduction to ammonium (DNRA) process in freshwater sediments of riparian zones

Dissimilatory nitrate reduction to ammonium (DNRA), an important intermediate process in the N-cycle, links N-compound oxidation and reduction processes. Hence, the oxic-anoxic interface would be the hotspot of the DNRA process. In freshwater ecosystems, the riparian zone is the most typical carrier of the oxic-anoxic interface. Here we report spatio-temporal evidence of a higher abundance and rate of DNRA in the riparian zone than in the open water sediments based on molecular and 15N isotopic-tracing technologies, hence signifying a hotspot for the DNRA process. These abudance and rates were significantly higher than those in open water sediments. 15N isotopic paring technology revealed that the DNRA hotspot promoted higher rates of N-compound oxidation (NO2-), reduction (NO3- and DNRA), and N2 production (anammox and denitrification) in the riparian zone than those in open water sediment. However, high-through sequencing analysis showed that the DNRA bacteria in the riparian zone and openwater sediments were insignificantly different. Network and correlation analysis showed that the DNRA abundance and rates were significantly positively correlated with TOM, TC/NH4+, and TC/NO2-, but not with the dominant genera (Anaeromyxobacter, Lacunisphaera, and Sorangium), which played different roles on the connection in the respective community networks. The DNRA process in the riparian zone could be driven mainly by the related environmental biogeochemical characteristics induced by anthropogenic changes, followed by microbial processes. This result provides valuable information for the management of riparian zones because anthropogenic changes in the riparian water table are expected to increase, inducing consequent changes in the reduction from NO3- to NH4+.