Multiyear changes in snowmelt phosphorus runoff with soil P drawdown or application of struvite to an organic forage crop

Abstract In this study, changes in P runoff losses from three field‐scale small watersheds in the Northern Great Plains region (Manitoba, Canada) under organic management were evaluated over a 7‐year period (2017–2023). A perennial forage crop was grown through this time period, either with biennial addition of a slow‐release fertilizer to maintain soil fertility (struvite, 4 site‐years) or without P amendment (drawdown, 8 site‐years). Annually measured soil P (Olsen P) strongly correlated with mass balance of P added as struvite or removed by crops and surface runoff. As a result, P limitation of forage yield was observed in the later years of the drawdown treatment. P loads and flow‐weighted mean concentrations (FWMCs) measured in snowmelt runoff after struvite application were compared to predicted values using published regional models developed with 82 site‐years of data from field‐scale small watersheds, with Olsen P, water yield, and percent surface cover as predictors. To evaluate response to struvite over a wider range of conditions, P runoff was measured in two additional watersheds under conventional annual grain production following struvite application (2 site‐years). Higher risk of P loss was observed with higher Olsen P. However, following struvite applications, the risk of P loss to snowmelt was consistently lower than predicted (between −0.13 and −0.85 mg L −1 for FWMC of total dissolved P) suggesting that the solubility of struvite in runoff water may be lower than for other forms of soil P also extractable as Olsen P.