Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/1999
Publication Date: N/A
Citation: POTE, D.H., DANIEL, T.C., NICHOLS, D.J., MOORE JR, P.A., MILLER, D.M., EDWARDS, D.R. 1999. SEASONAL AND SOIL-DRYING EFFECTS ON RUNOFF PHOSPHORUS RELATIONSHIPS TO SOIL PHOSPHORUS. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 63:1006-1012. Interpretive Summary: Reducing phosphorus (P) levels in runoff water is often recommended as the best way to control algae growth in streams and lakes. Previous studies have shown that soils with high P levels can be a major source of P in runoff water. We wanted to know if the amount of P released from soil is also affected by changes in season or initial soil moisture. Therefore, we compared results from a runoff event occurring in May, when initial soil moisture was 26.9%, to a runoff event in August when initial soil moisture was only 4.6%. In both seasons, plots with high STP produced the highest P levels in runoff water. Although soil test P (STP) (Mehlich-III method) did not change from May to August, P concentrations in runoff were about twice as high in August as in May. Distilled water extracted more P from dry soil than from moist soil, but when air-dried samples from May and August were compared, results from this STP method agreed with the Mehlich III method that STP levels had not changed. However, P-saturation of the soil was higher in August than May, following the sametrend as P concentrations in runoff. Because P loss in runoff can be a serious problem for water quality, these results provide important new information for scientists and watershed managers working to improve water quality while still maintaining high levels of agricultural production.
Technical Abstract: 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 tested a hypothesis that seasonal changes in field conditions (especially soil moisture) and 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) to produce 30 min of runoff, and filtered runoff samples were analyzed for DRP. Each soil sample was analyzed for water content, sieved (2 mm), and split into two subsamples. One subsample 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, but all soil subsamples were analyzed by Mehlich III and distilled water 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=28.5 mg/kg) and Mehlich III STP (mean=145 mg/kg) were not affected by season, but DRP concentration in August runoff (mean =1.05 mg/L) was almost double that in May (mean=0.57 mg/L), so the resulting correlations were affected. Water-extractable STP from field moist soil was higher in August (mean=23 mg/kg) than May (mean=16 mg/kg), and P saturation levels showed a similar trend. Runoff volumes were smaller in August, so season had little effect on mean DRP mass loss.