Unraveling multiple effects of environmental factors on nutrient release: Towards optimal conditions in agricultural drainage ditches.

Agricultural drainage ditches (ADDs) are widespread and serve as critical zones, playing a vital role in mitigating non-point source nitrogen (N) and phosphorus (P) pollution in farmland. However, limited information is known about the interaction relationship between multiple factors and nutrient release in multistage ADDs. Here, a comprehensive five-level agricultural drainage system (comprising field, sublateral, head, branch, and trunk ditches) was selected as the study model. We developed a simulation setup and applied the Box-Behnken Design (BBD) model to systematically examine the influence of various environmental factors on nutrient release dynamics. The results demonstrated that the developed regression model (p < 0.0001) reliably predicted total nitrogen (TN) and total phosphorus (TP) concentrations in multistage ADDs. Based on the response surface plots and contour lines, optimal release conditions for nutrients in multistage ADDs (5 days, 296 K, 1.03 mg/L N, 0.10 mg/L P) were determined through multi-objective optimization. We found that nutrient application exerted the most substantial influence on the release conditions. As field water was discharged into the ditches, the nutrient levels were decreased progressively. Overall, the N and P release processes in multistage ADDs were jointly influenced by environmental factors and ditch scale. These findings offer a solid theory for studying nutrient release and deposition in multistage ADDs, providing key insights for managing farmland ditches to reduce agricultural pollution.