Differences in Phosphorus Retention and Release in Soils Amended with Animal Manures and Sewage Sludge

Excessive levels of P in agricultural soils pose a threat to local water quality. This study evaluated (i) time‐dependent changes in soil P sorption (expressed as a phosphorus sorption index, PSI) and P availability (as resin P) during incubation (100 d) with poultry litter, cattle slurry, sewage sludge, or KH 2 PO 4 , added on a P‐equivalent basis (100 mg P kg −1 ), and (ii) the subsequent kinetics of P release, measured by repeated extractions with a mixed cation‐anion exchange resin. Soil exchangeable Ca and ammonium oxalate‐extractable Fe and Al were also determined at 100 d of incubation. The small decrease in P sorption in the litter and sludge treatments (25%), compared with that in the slurry and KH 2 PO 4 treatments (52%) between 20 and 100 d of incubation was attributed partly to the formation of new adsorption sites for P. Subsequent P release was described by a power equation: Resin P = a (extraction number) b , where the constants a and b represent resin P obtained with a single extraction and the rate of P release per resin extraction, respectively. On average, the rate of P release decreased in the order: KH 2 PO 4 and slurry > litter > sludge, and was inversely related to exchangeable Ca content of the incubated soils ( R 2 = 0.57). The slower rates of P release in the litter and sludge treatments ( P < 0.001) are attributed to the large values for exchangeable Ca (1050–2640 and 1070–2710 mg kg −1 , respectively) in these amended soils. Future research concerned with short‐term declines in environmentally harmful levels of P in recently amended soils should consider the differential effects of the amendments on soil P dynamics.