Rapid Incidental Phosphorus Transfers from Grassland

In Britain, frequent rainfall means that there is a high potential for rapid, direct (incidental) losses of phosphorus (P) to occur after fertilizer or manure application. However, despite the known contribution of P to the eutrophication of water bodies in Britain, such incidental transfers have received little experimental attention. To rectify this, we used lysimeter plots (each 3 × 10 m) to investigate incidental transfers in a composite of overland and lateral subsurface flow (0–27 cm) following the application of different P sources. The treatments used were triple super phosphate (TSP), dairy slurry (Slurry), an equal mix of TSP plus slurry (TSP + Slurry), and no P (Zero P). The treatments were applied to wet soil at a rate of 29 kg ha −1 In the following 169 h, 48.8 mm rainfall (intensity ≤3 mm h −1 ) resulted in total phosphorus (TP) exports between 1.8 and 2.3 kg ha −1 A single 4‐h period (with overland flow) accounted for 33 to 46% of overall loads from the P‐amended treatments. Concentrations in discharge from TSP + Slurry and TSP peaked at 11000 μg TP L −1 (67–68% as reactive P < 0.45 μm [RP < 0.45 ]). Slurry peaked at 7000 μg TP L −1 , 66% as particulate TP (>0.45 μm) and 20% as RP < 0.45 Even in subsurface flow, concentrations exceeded 3000 μg TP L −1 for all P‐amended treatments. Incidental TP concentrations in plot discharge were up to 110‐fold higher than those considered eutrophic in inland waters. We suggest that targeting short‐term management decisions for P applications is the most immediately viable method to mitigate P loss and benefit the environment.