Nitrogen cycling in China marginal seas: Progress and challenges

Marginal seas play a critical role in the earth's systems as hotspots of microbially-mediated nitrogen (N) cycling. Meanwhile, human activities and global changes strongly impact this land-ocean boundary. Understanding the key N cycling processes and their climatic feedback is crucial for predicting the status of earth in the future and developing global N-driven physical-biogeochemical models. Although N cycling processes in China marginal seas have received great attention, a systematic analysis over a large geographical scale from estuaries to the deep ocean is still missing. This gap hinders us from concretely determining the sources and fate of N and predicting their responses to changing environments. This review compiles and re-analyzes historical data of source terms, encompassing N fluxes from riverine inputs and atmospheric deposition, as well as N₂ fixation. We also examine sinks related to sedimentary N loss processes (denitrification and anammox), internal N cycling processes (uptake and nitrification), short-lived N intermediates, and nitrous oxide, by considering rates, spatiotemporal variability, and environmental factors. Finally, we outline future research directions pertaining to marine N cycling in China marginal seas.