Effects of Nitrate Exposure on Nitrate Reduction Processes in the Wetland Sediments from the Yellow River Estuary

Nitrate overloading in estuarine and coastal regions can lead to various environmental problems. However, limited information is available about nitrate reduction processes and their associated contributions to nitrogen removal in estuary environments. In this study, stable isotope tracing, high-throughput sequencing, and qPCR methods were used to explore the effects of different nitrate concentrations (CK, 100 μM, 200 μM, 400 μM, and 600 μM) on the nitrate reduction rates, microbial abundances, and community structures in wetland sediments from the Yellow River estuary. With nitrate concentration increasing, the denitrification rate increased from 1.08 to 5.97 nmol N g−1 h−1, while the anaerobic ammonium oxidation rates decreased from approximate 1.25 to 0.42 nmol N g−1 h−1. Diversity analyses based on high-throughput sequencing data indicated that the Shannon indexes were lower in the 400 and 600 μM nitrate-addition treatments than other treatments, and the 600 μM nitrate treatment has the lowest α-diversity. The β-diversity results showed that the bacterial community compositions were different between the ≤ 200 μM and ≥ 400 μM nitrate concentration treatments. The nitrate concentration can be divided into high (≥ 400μM) and low (≤ 200) groups based on multiple regression analyses. The co-occurrence network revealed that there was a large difference in the bacterial relationship between the two groups. Linear discriminant effect analysis showed that Gammaproteobacteria, Bacilli, and Actinobacteria were the indicator taxa in the high nitrate concentration group. Overall, these results indicate that nitrate is a key factor to influence the nitrate reduction rates and related microbial communities in estuarine environments.