Phosphorus forms in overland flow from agricultural soils representative of Mediterranean areas

Phosphorus (P) losses through overland flow (surface runoff) may contribute to eutrophication of water bodies. The main purpose of this work was to study P forms in overland flow (dissolved and particulate) to identify which can be potentially used by algae. To this end, rainfall on 17 representative soils from Mediterranean areas was simulated, and P forms in overland flow studied by chemical and sink (resin, iron oxide-impregnated paper strip) extraction; sequential chemical fractionation of the suspended sediments was also used to establish "operational pools with a differential capacity of P release. Total P (TP) in runoff ranged from 0.089 to 0.765 mg L-1 and was mainly related to suspended sediment (particulate P, 86% of TP on average). Iron oxide strip P, which is taken to be an estimate of algal-available P, accounted for 34% of TP on average in runoff samples; most of the P extracted by this sink was particulate P (68%). In most cases, FeO strip P was equivalent to dissolved reactive P (DRP) plus P extracted by NaOH and citrate-bicarbonate in suspended sediment (the more labile P fractions) ( Y=X, R-2 =0.82; P ). One can thus assume that Fe oxide extracts DRP, adsorbed P on sediments, and P related to highly soluble precipitated Ca phosphates, but it does not extract releasable P through reduction of sorbent surfaces or the organic P that can be mineralized in the bottom of water reservoirs, which must be taken in account to estimate the long-term algal-available P in runoff.