Edge-of-field runoff phosphorus loss from silage corn field and grazed grassland in the Fraser Valley, BC, Canada

Phosphorus (P) transport via rainfall-driven runoff is a primary driver of aquatic eutrophication, yet direct field-scale measurements in the Fraser Valley remain scarce. The objectives of this study were to monitor P losses via edge-of-field runoff during the non-growing season in two contrasting cropping systems [silage corn (Site 1) and grazed grassland (Site 2)] and to evaluate the efficacy of P saturation index (PSI) and water extractable P (Pw) in predicting observed P transport across multiple Rainfall events. Runoff was monitored between 2024 and 2026 using V-notch weirs and automated samplers, while grid soil sampling assessed environmental P indicators, including Pw and PSI. Total P (TP) concentrations in runoff at both sites consistently exceeded the 0.1 mg L -1 hyper-eutrophic threshold. Despite the lack of mineral fertilization, Site 2 exhibited significantly higher average concentrations (Dissolved reactive P (DRP): 0.29; Particulate P (PP): 0.52; TP: 0.95 mg L -1 ) compared to Site 1 (DRP: 0.13; PP: 0.17; TP: 0.42 mg L -1 ). Transport dynamics were system-specific with Site 1 exhibiting incidental spikes immediately following fall manure applications, whereas Site 2 transitioned from DRP to PP dominance as seasonal cattle trampling disturbed saturated soils. The study confirmed that Pw and PSI are robust, interchangeable predictors of dissolved P loss, accurately identifying Site 2 as high-risk with Pw = 7.8 mg kg -1 and PSI = 17.4%. Spatial mapping revealed topographic depressions as critical hydrological hotspots in silage corn fields, where P accumulates due to tillage and water erosion (Pw = 2.43 mg kg -1 ). These findings underscore that agricultural runoff in the Fraser Valley can pose a severe environmental risk and highlights the urgency for spatially targeted management strategies to mitigate P export to regional water bodies.