SOIL-PHOSPHORUS AS A POTENTIAL NONPOINT-SOURCE FOR ELEVATED STREAM PHOSPHORUS LEVELS

Eroded soils, as a potential nonpoint P source, have come under scrutiny, particularly in agricultural watersheds. Surface water and groundwater in the Tualatin River Basin (TRB) of Northwestern Oregon have maximum concentrations of 2.1 and 2.6 mg P/L, respectively. We hypothesized that TRB soils are a major P source in the basin because of their high native soil P concentrations and soil P adsorption characteristics. We sampled eight soil series from both lowland and upland positions in the TRB, with both andic and nonandic mineralogy. All soils sampled had high total P concentrations (660-2304 mg P/kg soil). High average extractable P (Bray P1, 74 mg P/kg soil, and ammonium bicarbonate, 35.7 mg P/kg soil) concentrations were found primarily in lowland, nonandic soils. There was no significant difference between P sorption maxima; however, P affinity was significantly higher in upland soils (0.19 L/mg P) than in lowland soils (0.04 L/mg P). Upland, andic soils, due to their erosion potential and high TP levels (average of 1889 mg P/kg sea), could be a P source for surface waters. Lowland, nonandic soils, due to their high extractable P concentrations and lower P sorption affinity, could be a source for both surface and groundwater. Thus, all soils studied, comprising approximately 50% of TRB soils, are potential nonpoint P sources in this watershed, indicating that native soil P is a potential source of nonpoint P pollution.