Phosphorus in Soil and Runoff Following Swine Manure Injection with a Low-Disturbance Applicator.

Injection of liquid swine manure disturbs surface soil, making it susceptible to erosion. Runoff from treated lands can transport nutrients and pathogens to surface waters. Our objective in this field study was to determine the effect of two swine manure application methods on phosphorus (P) fate and transport in a corn (Zea mays L.) production system with and without a cereal rye cover crop. The experimental design (RCB; four replications) included six treatments: i) no manure, with and without a cover crop; ii) conventional knife injection of manure, with and without a cover crop; and iii) low-disturbance injection of manure, with and without a cover crop. Manure application supplied 168 kg N ha^-1 and 72 kg P ha^-1. Subsamples of runoff generated with a rainfall simulator were collected for analysis of dissolved reactive P, total P, and total suspended solids. Rainfall was applied 8 days after manure application (early November) and in mid-April. Soil P availability was assessed with anion exchange membranes inserted vertically to a 30-cm depth within and perpendicular to the row. Phosphorus concentrations in runoff declined during the three-week period after application. Losses of DRP were greater in both the fall and spring following low-disturbance injection of manure. However, manure application method had no effect on TP and sediment loads in runoff in either season. The rye cover crop reduced fall TP losses from plots with manure applied by either method. In the spring, DRP losses were significantly higher from plots with the recently killed cover crop, but TP and sediment losses were not affected. Manure application significantly increased available P in the soil. The highest P levels (21-27 μg P cm^-2) were detected in the 20-30 cm layer of plots with conventional manure application, while in plots with low-disturbance manure application, the highest P levels (25-27 μg P cm^-2) were detected in the 5-20 cm layer. Results suggest that subsurface application of swine manure into a standing cover crop minimizes P losses and provides optimum P availability to the subsequent corn crop.