Nitrogen–Phosphorus Pollution Dynamics and Export Processes of Anthropogenic Polders in the Middle–Lower Yangtze River: A Regional Review

Polders are typical semi-artificial and human-dominated ecosystems widely distributed in the middle and lower reaches of the Yangtze River Basin. They serve as important sinks and sources of nitrogen (N) and phosphorus (P) in agricultural watersheds. Long-term intensive human intervention substantially alters the hydrological and biogeochemical processes of polder ecosystems, resulting in complex and uncertain effects on water quality that remain insufficiently understood. This study conducts a systematic literature review and narrative synthesis of evidence on N and P transport in polders across the middle–lower Yangtze River plain. The evidence is derived from field monitoring, plot experiments, and numerical simulations. The review focuses on the spatiotemporal patterns of nutrient variation, sink–source conversion functions of ditches and small ponds, drivers of nutrient loss, and current research bottlenecks under artificial sluice-pump regulation. The synthesized results indicate that polders exhibit a distinctive nutrient transport pattern characterized by dispersed in situ retention under conventional water management and concentrated pulse export during drainage events. Artificial sluice-pump operation drives episodic nutrient export throughout the crop growth period, imposing persistent pressure on the water quality of downstream rivers and lakes. N and P transformations are jointly controlled by natural hydrological fluctuations and human activities. Within agricultural lands of polders, fertilizers account for 73.3% of total nitrogen inputs and 87.9–93.5% of total phosphorus inputs. Crop harvesting and regulated drainage constitute the two dominant pathways for nutrient export. Hydraulic regulation prolongs water residence time in polder ditches and ponds, resulting in retention efficiencies of 52–65% for allochthonous N and P. However, seasonal flooding and waterlogging can induce sediment hypoxia and endogenous nutrient release, thereby causing secondary internal pollution and increasing the eutrophication risk of adjacent receiving water bodies. Three major research gaps are identified: insufficient long-term continuous multi-indicator monitoring data, limited model applicability for simulating human-regulated hydrology–nutrient coupling, and poorly defined critical thresholds for polder sink–source functional reversal. This regional systematic review advances the understanding of human–hydrology–nutrient coupling mechanisms in Yangtze River polder systems. It also provides targeted theoretical support for agricultural non-point source pollution mitigation and water environment management in floodplain agricultural areas.