RAINFALL FREQUENCY AND NITROGEN AND PHOSPHORUS RUNOFF FROM SOIL AMENDED WITH POULTRY LITTER

Repeated land application of manure has enriched runoff N and P in som e areas. Here, more restrictive manure management guidelines may be ad opted. The main factors affecting N and P runoff from land receiving m anure include the rate, method, and timing of application. Of these fa ctors, least information is available on timing; thus, the effect of r ainfall frequency and timing (7-35 d at 2.54 cm h(-1)) after poultry l itter application (incorporated into surface 5 cm of soil at 10 Mg ha( -1); i.e., 380 kg N and 160 kg P ha(-1)) on N and P concentrations in runoff was investigated for 10 Oklahoma soils, Runoff N and P concentr ations decreased with 10 successive rains, starting 7 d after litter a pplication. Although the decrease in N concentration was independent o f soil type, decreases in dissolved (DP) and bioavailable P (BAP) were related to percent saturation of soil P sorption sites (r(2) of 0.97 and 0.62, respectively). Increasing the time between litter applicatio n and rainfall from 1 to 35 d reduced total N from 7.54 to 2.34, ammon ium-N (NH4-N) 5.53 to 0.11, DP 0.74 to 0.45, and BAP 0.99 to 0.65 mg L -1, by leaching and volatilization of N and sorption of P. Nitrate-N ( NO3-N) concentrations were unaffected by rainfall frequency and timing . Runoff N and P may he reduced by avoiding litter applications during periods of high rainfall probabilities, with these reductions greater for high than low P sorbing soils.