Sorption site competition determines phosphorus availability of agrifood residues

Phosphorus (P) extractability, and thus plant-availability, in processed agrifood residues incorporated in soil is not directly related to the extractability before the incorporation. However, the release of P has not been demonstrated over seasons, and the mechanisms involved are not well understood. We identified the fate of P fractions in manure and sewage sludge processed with current methods after incubation for two weeks, 2.5 months and 12 months in sandy loam or clay, using a modified Hedley fractionation scheme. We also identified the sorption of P to soil, and the determinants. The changes in readily extractable P since the incorporation ranged in sandy loam from - 23% to + 88% and in clay from - 49% to + 31%. Incorporation of manure and synthetic fertilizer led in soil to a clearly lower P extractability than sewage sludge. Precipitation of P in sewage with molar Fe/P 1.6 increased P extractability relative to biological precipitation (Fe/P 0.2) and molar Fe/P 9.0. Readily extractable P increased most with sewage sludge hygienized by acid and oxidizer, and next by anaerobic digestion only in clay but not with manure. The differences in P extractability were explained by competition for P sorption of varying capacity and intensity. The amount of added organic matter in residues, rather than of iron oxides and hydroxides, appeared as the consequently explaining factor to P sorption, increasing the sorption irrespective of the processing method.