The Transport of Bioavailable Phosphorus in Agricultural Runoff

Abstract

Bioavailable P (BAP) in agricultural runoff represents P potentially available for algal uptake and consists of soluble P (SP) and a variable portion of participate P (PP). Evaluation of the impact of agricultural management on BAP in runoff will aid assessment of the resultant biological productivity of receiving water bodies. Soluble P, PP, and bioavailable PP (BPP) (estimated by NaOH extraction) were determined over a 5‐yr period in runoff from 20 unfertilized and fertilized, grassed, and cropped watersheds in the Southern Plains. Soluble P, BPP, and BAP loss in runoff was reduced by practices minimizing erosion and runoff, with respective mean annual amounts ranging from 237 to 122, 1559 to 54, and 1796 to 176 g P ha −1 yr −1 (for peanut‐sorghum [ Arachis hypogaea L.‐ Sorghum bicolor (L.) Moench] and native grass watersheds, respectively). However, as vegetative cover improved, BAP (SP plus BPP) comprised a larger portion of total P (TP) loss (29% for peanut‐sorghum and 88% for native grass). This results from an increasing contribution to BAP of SP (13% for peanut‐sorghum and 69% for native grass watersheds) and BPP to PP (26% for peanut‐sorghum and 69% for native grass watersheds). Clearly, P bioavailability is a dynamic function of physiochemical processes controlling erosion, particle size enrichment, P desorption‐dissolution reactions, and plant residue breakdown, in addition to soil and fertilizer P management. Hence, the change in trophic state of a water body may not be adequately reflected by TP inputs only. To more reliably evaluate the biological response of a water body to agricultural P inputs, particularly from conservation tillage practices, it may be necessary to determine BAP in runoff.