Differences in Phosphorus Retention and Release in Soils Amended with Animal Manures and Sewage Sludge
Siddique, Muhammad Tariq | Robinson, J Stephen
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 KH₂PO₄, 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 KH₂PO₄ 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: KH₂PO₄ 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.
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