Linking abundance and community of microbial NO-producers and NO-reducers with enzymatic NO production potential in a riparian zone.

As aquatic-terrestrial ecotones, riparian zones are hotspots not only for denitrification but also for nitrous oxide (NO) emission. Due to the potential role of nosZ II in NO mitigation, emerging studies in terrestrial ecosystems have taken this newly reported NO-reducer into account. However, our knowledge about the interactions between denitrification activities and both NO-producers and reducers (especially for nosZ II) in aquatic ecosystems remains limited. In this study, we investigated spatiotemporal distributions of in situ NO flux, potential NO production rate, and potential denitrification rate, as well as of the related genes in a riparian zone of Baiyangdian Lake. Real-time quantitative PCR (qPCR) and high-throughput sequencing targeted functional genes were used to analyze the denitrifier communities. Results showed that great differences in microbial activities and abundances were observed between sites and seasons. Waterward sediments (constantly flooded area) had the lowest NO production potential in both seasons. Not only the environmental factors (moisture content, NH content and TOM) but also the community structure of NO-producers and NO-reducers (nirK/nirS and nosZ II/nosZ I ratios) could affect the potential NO production rate. The abundance of the four functional genes in the winter was higher than in the summer, and the values all peaked at the occasionally flooded area in the winter. The dissimilarity in community composition was mainly driven by moisture content. Altogether, we propose that the NO production potential was largely regulated by the community structure of NO-producers and NO-reducers in riparian zones. Increasing the constantly flooded area and reducing the occasionally flooded area of lake ecosystems may help reduce the level of denitrifier-produced NO.