Agricultural drainage networks as non-point phosphorus pollution control systems: scale-dependent retention and primary drivers

Agricultural drainage networks increasingly function as distributed pollution control systems for mitigating non-point source phosphorus (P) export to aquatic ecosystems. However, the spatial heterogeneity of P retention and its specific drivers across varying ditch orders remain poorly understood, as most studies focus on isolated, single-order segments. Here, we conducted a meta-analysis evaluating apparent total phosphorus (TP) and phosphate concentration removal efficiencies across sublateral, head, and branch ditches. Branch ditches exhibited the highest mean apparent concentration removal efficiencies, with TP removal at 17 ± 32%, 16 ± 28%, and 39 ± 37%, and phosphate removal at -8 ± 73%, 11 ± 43%, and 39 ± 32% in sublateral, head, and branch ditches, respectively. While influent P concentration emerged as the strongest predictor of apparent concentration removal across all ditch orders, secondary predictors showed scale-dependent variation. Physical and structural characteristics were relatively more influential in sublateral and head ditches, whereas hydrodynamic variables were relatively more important in branch ditches. Land-use systems and crop-cycle stages were also associated with apparent concentration removal, with higher efficiencies generally observed in non-paddy systems compared to paddy systems and during non-growing rather than active growing periods. Ditch modifications were associated with enhanced apparent TP concentration removal in sublateral and head ditches but showed limited effects in branch ditches. Together, these scale-dependent patterns provide a preliminary basis for spatially differentiated management. Structural or biogeochemical measures may warrant further evaluation in smaller ditches, whereas hydraulic regulation may be more relevant in larger branch ditches. This synthesis provides a quantitative basis for developing and testing targeted strategies to improve P attenuation across agricultural drainage networks.