FORMS AND CONCENTRATION OF PHOSPHORUS IN DRAINAGE WATER OF 27 TILE-DRAINED SOILS

In most mineral soils, P leaching is rarely viewed as an important env ironmental issue. However, P accumulation and decreased P sorption cap acities in surface horizons of long-term fertilized soils may increase downward P movement, The objective of this study was to measure the c oncentration and characterize the P forms in drainage waters from nine soil series widely differing in clay content. Twenty-seven sites were sampled in 1994 and 1995 from an intensively cropped area of the prov ince of Quebec, Canada, Drainage waters were characterized for their t otal P (TP), dissolved reactive P (DRP), dissolved organic P (DOP), an d total particulate P (TPP) contents. The Quebec surface water quality standard of 0.03 mg TP L-1 was exceeded in 14 out of 27 sites in 1994 but only in 6 out of 25 sites in 1995. Of the 14 sites exceeding 0.03 mg TP L-1 in 1994, 10 were clayey soils. Under these circumstances, m ore than 50% of the TP was as TPP whereas DOP forms represented <30%, In 1995, TPP forms accounted for, on average, <50% of TP and DOP accou nted for more than 40% of the TP concentrations. This study suggests t hat flat clayey soils of medium to rich P status may be particularly a t risk of exceeding water quality standards in subsurface runoff. Phos phorus losses in particulate form may be important in subsurface runof f from clayey soils when weather conditions favor rapid Bow through cr acks or macropores.