Seasonal and Soil-Drying Effects on Runoff Phosphorus Relationships to Soil Phosphorus

Controlling phosphorus levels in runoff is often recommended as the best way to minimize eutrophication of streams and lakes. Previous research has shown that increased concentrations of dissolved reactive P (DRP) in runoff from grassland are highly correlated to increased soil test P (STP) levels. We conducted an experiment to investigate the hypothesis that seasonal changes 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 wide range of STP were randomly divided into two groups. In May (wet season), soil samples were taken from each plot in the first group, 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 analyzed for H₂O content, sieved (2 mm), and split into two subsamples. One subsample from each plot was kept field-moist at 4°C, and the other was air dried. Phosphorus saturation was determined only on air-dry soil, but all soil subsamples were analyzed by Mehlich III and distilled H₂O methods. In August (dry season), the second group of plots received the same treatment. 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 = 28.5mg kg −1) and Mehlich III STP (mean = 145mg kg −1) were not affected by season, but DRP concentration in August runoff (mean = 1.05mg L −1) was almost double that in May (mean = 0.57mg L −1), so the resulting correlations were affected. Water-extractable STP from field-moist soil was higher in August (mean = 23mg kg −1) than in May (mean = 16mg 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.