Nitrogen cycling in aquatic environments of China: Progress and future challenges

Aquatic environments are hot spots of nitrogen (N) cycling and nitrous oxide (N₂O) emission. Understanding key N biogeochemical processes and N₂O dynamics is critical for uncovering the responses of aquatic environments to human activities. In this study, we summarized the development of techniques and approaches for determination of N cycling processes and N₂O sources in aquatic environments of China. We also reviewed the main N transformation processes and associated biogeochemical controls in aquatic environments of China (e.g., constructed wetlands, paddy fields, lakes, rivers, aquaculture ponds, and estuaries and coasts). In addition, we further synthesized N₂O dynamics in these aquatic environments. The results suggested that N cycling processes have been greatly concerned in various aquatic environments of China, whereas more comprehensive analyses of N cycling processes are still sparse for projecting N fates and dynamics in response to changing environments, largely limiting the implementation of N management strategy of China. Aquatic environments of China may be the important sources of atmospheric N₂O, but the production and consumption processes of N₂O need to be further studied for effective greenhouse gas mitigation. We also argued that it is urgently necessary to incorporate microbial traits into biogeochemical ecosystem modeling to improve the estimation reliability of N cycling and N₂O dynamics. This review highlights the importance of N cycling and N₂O dynamics, as well as associated environmental implications, and presents the directions of future research on N cycling in aquatic environments of China.