Seasonal and soil-drying effects on runoff phosphorus relationships to soil phosphorus

Controlling phosphorus levels in runoff is often recommended as the best na y to minimize eutrophication of streams and lakes. Previous research has sh own that increased concentrations of dissolved reactive P (DRP) in runoff f rom grassland are highly correlated to increased soil test P (STP) levels. We conducted an experiment to investigate the hypothesis that seasonal chan ges in field conditions (especially soil moisture) along with the practice of air-drying soil samples prior to analysis may affect such correlations. Grass plots with a aide range of STP were randomly divided into two groups. In May (wet season), soil samples were taken from each plot in the first g roup, simulated rain was applied (75 mm h(-1)) to produce 30 min of runoff, and filtered runoff samples were analyzed for DRP. Each soil sample was an alyzed for H2O content, sieved (2 mm), and split into two subsamples. One s ubsample from each plot was kept field-moist at 4 degrees C, and the other was air dried. Phosphorus saturation was determined only on air-dry soil, b ut all soil subsamples were analyzed by Mehlich III and distilled H2O metho ds. In August (dry season), the second group of plots received the same tre atment. All correlations of STP to runoff DRP were significant (P < 0.01), regardless of season or STP method. Water-extractable STP from air-dry soil (mean = 28.5 mg kg(-1)) and Mehlich III STP (mean = 145 mg kg(-1)) were no t affected by season, but DRP concentration in August runoff (mean = 1.05 m g L-1) was almost double that in May (mean = 0.57 mg L-1), so the resulting correlations were affected. Water-extractable STP from field-moist sod was higher in August (mean = 23 mg kg(-1)) than in May (mean = 16 mg kg(-1)), and P saturation levels showed a similar trend. Runoff volumes were smaller in August, so season had little effect on mean DRP-mass loss.