An Innovative Approach to Estimate Bioavailable Phosphorus in Agricultural Runoff Using Iron Oxide-Impregnated Paper

The transport of bioavailable phosphorus (BAP) in agricultural runoff can accelerate the eutrophication of receiving water bodies. Bioavailable P is comprised of dissolved P (DP) and bioavailable particulate phosphorus (BPP). Although several algal assays and chemical extractions have been proposed to estimate BAP, procedural and theoretical limitations have restricted their widespread use. This study evaluates the use of Fe oxide-impregnated paper strips (Fe-oxide strips) to estimate the potential BAP content of runoff from 20 agricultural watersheds in the Southern Plains during 1988 to 1990. In the proposed method, BAP and DP are determined by shaking 50 mL of unfiltered or filtered runoff, respectively, with one Fe-oxide strip for 16 h. Phosphorus is removed from the strip by 0.1 M H₂SO₄ and measured, with BPP calculated as the difference between BAP and liP. Growth of Selenastrum capricornutum was related (r² = 0.63–0.96) to the BPP content of runoff sediment in 2- to 29-d incubations. Bioavailable PP estimated by the proposed strip method and more widely used NaOH extraction, closely followed a 1:1 relationship. Averaged for all runoff events, strip DP was 90 and 75% of DP determined by standard direct methods, for watersheds with and without tillage, respectively. The lower estimates of strip DP may be partially attributed to a greater transport of acid-hydrolyzable organic and condensed DP in runoff from no-till compared to tilled watersheds. Acting as a P sink, the strip method may have a stronger theoretical basis than chemical extraction in estimating algal-available P in agricultural runoff. In addition, the simplicity of the proposed Fe-oxide strip method will allow BAP estimation with limited resources and may facilitate evaluation of the impact of agricultural runoff on the biological productivity of receiving waters.