ASSESSING PHOSPHORUS BIOAVAILABILITY IN AGRICULTURAL SOILS AND RUNOFF

Bioavailable phosphorus (BAP) transported in agricultural runoff can a ccelerate surface water eutrophication. Although several algal assays and chemical extractions have been proposed to estimate BAP, procedura l and theoretical limitations have restricted widespread BAP measureme nt. Thus, a routine method was developed to estimate BAP, which uses i ron oxide-impregnated paper strips (Fe-oxide strips) as a P-sink for B AP in runoff. In the proposed method BAP is determined by shaking 50 m L of unfiltered runoff with one Fe-oxide strip for 16 h. Phosphorus is removed from the strip by 0.1 M H2SO4 and measured. The BAP content o f runoff from 20 agricultural watersheds in the Southern Plains was re lated to the growth of P-starved algae incubated for 29 d with runoff as the sole source of P. Acting as a P sink, Fe-oxide strips may have a stronger theoretical basis than chemical extraction in estimating BA P. The method may also have potential use as an environmental soil P t est to indicate soils liable to enrich runoff with sufficient P to acc elerate eutrophication. Bioavailable P loss in runoff was lower from n o till (438 g ha-1 yr-1) than from conventional till (1288 g ha-1 yr-1 ). Kinetic and enrichment ratio approaches accurately predicted (r2 of 0.93) BAP transport in runoff during 1988 to 1990. Use of the Fe-oxid e strip method will facilitate estimation of BAP transport in runoff a nd thereby, improve assessment of the resultant impact on the biologic al productivity of receiving surface waters.