The importance of intra-management practice variability in sediment and phosphorus loss from agricultural fields

Eutrophication and sedimentation are pervasive challenges in the Chesapeake Bay watershed. Shallow-disk manure injection is a promising technology to mitigate phosphorus (P) losses in runoff while maintaining the water quality benefits of no-till, including reducing particulate P and sediment losses. However, its effectiveness as a P mitigation strategy is not fully understood, as hydrologic variability exists across spatial and temporal scales, confounding the results of field studies seeking to quantify the benefits of shallow-disk injection. In this study, we regressed loads of total solids (TS), total P (TP), dissolved P (DP), and particulate P (PP) against flow depths to evaluate the effectiveness of shallow-disk manure injection versus broadcasting for reducing P and sediment losses. Overland and subsurface flow from twelve plots in central Pennsylvania were measured and sampled for TS and P from January 2013 to May 2017. The plots received manure via either surface broadcasting or shallow-disk injection for three years, followed by two years of broadcasting across all plots. Load-discharge (L-Q) relationships were developed to determine how P and TS losses changed with increasing flow. L-Q analyses revealed dilution of all P constituents and near-chemostatic behavior for TS for both application methods. However, shallow-disk injection was more effective in promoting dilution of DP, and to a lesser extent, TP, with no difference between application methods for TS and PP. Intra-practice variability was largely dependent on relative contributions from overland and subsurface flow, due to increased dilution from subsurface flow. Overall, the results suggest that shallow-disk injection appears to be an effective practice to reduce TP and DP losses without negating the benefits of no-till.