Research Progress on Carbon and Nitrogen Migration, Transformation, Monitoring, and Engineering Regulation in Soil–Water Environmental Systems

Carbon and nitrogen are two key elements driving biogeochemical cycles in soil-water environmental systems. Their transport and transformation along the soil-river-reservoir-estuary continuum directly affect water eutrophication, organic carbon burial fluxes and regional eco-environmental security. Based on two representative studies—the effect of water-sediment regulation (WSR) of the Xiaolangdi Reservoir on suspended sediment and organic carbon in the Yellow River, and a systematic comparison of factors affecting soil nitrate-N and ammonium-N extraction—this review summarizes: (1) the power-function relationship between suspended sediment concentration and flow velocity; (2) the differential effects of WSR on particulate organic carbon (POC), dissolved organic carbon (DOC) and black carbon (BC); (3) the significant influences of six factors (air drying, sieve size, extracting reagent and concentration, solution-to-soil ratio, shaking time, and storage conditions) on soil mineral N determination; and (4) research trends in non-point source pollution control from the perspective of carbon-nitrogen coupling. Prospects for high-resolution monitoring, isotope tracing and model simulation are also discussed.