Suspended soil as a source of potentially bioavailable phosphorus in surface runoff waters from clay soils

The contribution of suspended soil as a source of potentially bioavailable P in runoff water was investigated by anion exchange resin (AER) extraction. The samples were also analysed for total P (TP) and dissolved reactive P (DRP). Particulate P (PP) was calculated by subtracting DRP from TP, and reversibly adsorbed particulate P (PPi) by subtracting DRP from AER-P. The material consisted of 154 surface runoff samples, collected from four arable clay soils in southern Finland. Total P and PPi correlated closely with TSS, but DRP did not. When the TSS concentration increased, the increasing proportion of AER-P consisted of PPi. In runoff low in TSS (0.2–0.5 g l−1), PPi constituted 12–23% of AER-P; the dominant fraction (77–88%) of AER-P being DRP. When the TSS concentration increased, PPi became an important contributor to potentially bioavailable P, accounting for 35–69% of AER-P when TSS was 1.0–2.5 g l−1. The average amount of PPi in suspended soil material ranged from 39 to 165 mg kg−1 at different sites, depending on soil P status. If PPi is not taken into account, P bioavailability in turbid runoff may be severely underestimated.