Mitigating Phosphorus Flux from Agricultural Tile Drainage into Eutrophic Headwater Streams: A Reactive Filter Barrier Approach Under Variable Hydrological Loading

Abstract

Excessive phosphorus (P) export via subsurface tile drainage is a principal driver of eutrophication in agricultural headwater catchments, yet cost-effective interception strategies remain poorly operationalized at the field scale. This study evaluated the P-retention efficacy of calcium-amended biochar and iron-coated sand reactive filter barriers installed in tile drainage outlets across three intensively farmed watersheds in the Po Valley, northern Italy, over a 24-month monitoring period. Dissolved reactive phosphorus (DRP) and particulate phosphorus (PP) fluxes were quantified under contrasting hydrological loading regimes using high-frequency turbidity proxies and grab sampling. Reactive barriers reduced cumulative DRP export by 61–78% during peak-flow events, with sorption capacity governed primarily by initial soil legacy P concentrations and antecedent moisture conditions. These findings provide a scalable, field-validated framework for targeting P interception in tile-drained agroecosystems.