Effect of broadcast manure on runoff phosphorus concentrations over successive rainfall events

Concern over eutrophication has directed attention to manure management effects on phosphorus (P) loss in runoff. This study evaluates the effects of manure application rate and type on runoff P concentrations from two, acidic agricultural soils over successive runoff events. Soils were packed into 100- × 20- × 5-cm runoff boxes and broadcast with three manures (dairy, Bos taurus; layer poultry, Gallus gallus; swine, Sus scrofa) at six rates, from 0 to 150 kg total phosphorus (TP) ha^-1. Simulated rainfall (70 mm h^-1) was applied until 30 min of runoff was collected 3, 10, and 24 d after manure application. Application rate was related to runoff P (r² = 0.50-0.98), due to increased concentrations of dissolved reactive phosphorus (DRP) in runoff; as application rate increased, so did the contribution of DRP to runoff TP. Varied concentrations of water-extractable phosphorus (WEP) in manures (2-8 g WEP kg^-1) resulted in significantly lower DRP concentrations in runoff from dairy manure treatments (0.4-2.2 mg DRP L^-1) than from poultry (0.3-32.5 mg DRP L^-1) and swine manure treatments (0.3-22.7 mg DRP L^-1). Differences in runoff DRP concentrations related to manure type and application rate were diminished by repeated rainfall events, probably as a result of manure P translocation into the soil and removal of applied P by runoff. Differential erosion of broadcast manure caused significant differences in runoff TP concentrations between soils. Results highlight the important, but transient, role of soluble P in manure on runoff P, and point to the interactive effects of management and soils on runoff P losses.