Load-discharge relationships reveal the efficacy of manure application practices on phosphorus and total solids losses from agricultural fields

Eutrophication and sedimentation are pervasive challenges in many agricultural watersheds. Recent research has promoted shallow-disk manure injection as a means of mitigating phosphorus (P) losses in runoff while maintaining the water quality benefits of no-till, such as reduced particulate P and sediment losses. However, the precision and accuracy of field studies seeking to quantify the effectiveness of shallow-disk injection as a P mitigation strategy are substantially constrained by hydrologic variability across spatial and temporal scales. In this study, overland and subsurface flow from twelve plots in central Pennsylvania (PA) were measured and sampled for all P constituents and total solids (TS) from January 2013 to May 2017. We regressed loads of total P (TP), dissolved P (DP), particulate P (PP), and TS against flow depths to evaluate how P and TS losses changed with increasing flow. The results revealed dilution of all P constituents and near-chemostatic behavior (little change in concentration with change in flow) for TS for both application methods. Shallow-disk injection was found to be more effective than broadcasting in promoting dilution of DP, and to a lesser extent, TP. In contrast, the broadcast plots showed stronger dilution patterns than did the injection plots for PP, and there was no difference between application methods for TS. Variability among plots within each manure application practice was largely dependent on relative contributions of overland and subsurface flow due to increased dilution of P by subsurface flow. Overall, shallow-disk injection appears to be an effective practice to reduce DP and TP losses without negating the erosion-reducing benefits of no-till.