Seasonal variations and anthropogenic dredging drive dominant mechanism shifts in phosphorus retention in agricultural drainage ditches.

Although channel conditions in agricultural drainage ditches vary substantially with seasonal changes and dredging activities, their effects on phosphorus retention mechanisms remain poorly understood. This study combined tracer experiments with the One-dimensional Transport with Inflow and Storage (OTIS) model to investigate phosphorus retention under different channel conditions in the Fourth Main Drainage Ditch of the Hetao Irrigation District, China. Results showed that from summer to autumn and into the spring following dredging, the overall retention rate of soluble reactive phosphorus (SRP) decreased, while the contribution of the main channel increased progressively and became dominant. During periods with dense vegetation and a well-developed storage zone, biological uptake and assimilation likely played a major role in phosphorus retention. In contrast, reduced vegetation cover, increased flow velocity, and alterations in channel ecological structure following dredging may have contributed to the decline in phosphorus retention capacity and the increased contribution of the main channel, while enhancing the relative importance of adsorption processes. These findings suggest that seasonal variations and anthropogenic dredging can influence phosphorus retention patterns and modify the relative importance of different retention mechanisms in agricultural drainage ditches. The results highlight the importance of maintaining ecological storage functions and provide a scientific basis for optimizing ditch management, improving water quality, and mitigating agricultural nonpoint-source pollution.