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

Eroded soils, as a potential nonpoint P source, have come under scruti ny, particularly in agricultural watersheds. Surface water and groundw ater in the Tualatin River Basin (TRB) of Northwestern Oregon have max imum concentrations of 2.1 and 2.6 mg P/L, respectively. We hypothesiz ed that TRB soils are a major P source in the basin because of their h igh 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, nonan dic soils. There was no significant difference between P sorption maxi ma; 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/k g sea), could be a P source for surface waters. Lowland, nonandic soil s, due to their high extractable P concentrations and lower P sorption affinity, could be a source for both surface and groundwater. Thus, a ll soils studied, comprising approximately 50% of TRB soils, are poten tial nonpoint P sources in this watershed, indicating that native soil P is a potential source of nonpoint P pollution.