Manure Phosphorus and Soil Test P Effects on Runoff P from Simulated Rain.

Manure application to cropland can contribute to runoff losses of nutrients and eutrophication of surface waters. We conducted a rainfall simulation study to assess the effects of dairy heifer diet P, soil test P, and manure incorporation on runoff P losses from two successive rains. Bedded manure (21% DM) from dairy heifers offered diets with or without supplemental P was either surface-applied or incorporated on a silt loam soil with either optimum or excessively high soil test P packed into 1 x 0.2-m sheet metal pans. Equal manure application rates supplied the equivalent of either 41 or 52 kg P/ha for the low and high P manures. We measured runoff volume, total (TP) and dissolved reactive P (DRP), and total solids (TS) in runoff collected for 30 min after runoff initiation from rain events (70 mm/h) one day and three days after manure application. Manure incorporation reduced TP and DRP concentration and load by 85-90% compared to surface application. Manure P and soil test P had minimal effects on P concentrations in runoff from Rain 1 because of counteracting effects on TS, but the P concentration of solids in runoff was decreased from low P manure (surfaced-applied) and from lower soil test P (incorporated and non-manured treatments). Manure and soil P effects were more pronounced in the runoff from Rain 2; DRP concentration and load from the combination of low manure P and optimum soil test P in the incorporated treatment did not differ from the no-manure control. Overall, concentrations of TS, TP, and DRP in runoff from Rain 2 were 25-75% lower than from Rain 1. These results show that large reductions in P runoff losses can be achieved by incorporation of manure, and that additional benefits can be gained by reducing diet P and avoiding soils with excessive P.