Three chemicals: aluminum sulfate (alum), ferrous sulfate (FeSul) and calcium oxide (CaO), were used to stabilize the phosphorus (P) in fresh dewatered anaerobically digested sewage sludge (FSS) to produce alum-SS, FeSul-SS, and CaO-SS, respectively. Two incubation experiments were set up to test the efficiency of P stabilization in the sludge as a means of maximizing the land's capacity to safely receive this waste material. Olsen-P, water-soluble P (WSP) and water-soluble Ca, Fe, Al, Mg, K, and S were monitored during the incubation period. Phosphorus phytoavailability in these sandy soil amended with different sludge materials or KH2PO4, at rates of 50 to 250 mg P kg-1, were probed by a newly developed rapid bioassay which corresponded to the inorganic P(Pi) content of tested plant. Increased P application rates resulted in increased WSP and Olsen-P values after a 110-day incubation period, but the increase was dependent on the sludge pretreatments: at the same amount of applied P, the Olsen-P and WSP values in the sludge materials were ranked as follows: FSS >> CaO-SS > alum-SS > FeSul-SS. At the end of incubation, WSP in the soil amended with Fe-Sul-SS did not increase over the control even at the highest P rates. At the same time, water-soluble Al and Fe decreased and water-soluble Ca, S and K increased in the soil amended with FeSul-SS and alum-SS. The order of P phytoavailability with the same rate of P application was KH2PO4 >> alum-SS ≥ BSC ≥ CaO-SS > FSS >> FeSul-SS. Phosphorus phytoavailability was positively related to P rates and WSP content for each soil, but the corresponding dependency coefficients differed considerably. Overall, these results indicate that P solubility and phytoavailability can be either enhanced or reduced by sludge pretreatments, and the soil capacity to safely receive high loads of sludge can be increased significantly by sludge pre-stabilization with FeSul. This approach is expected to benefit the practice of land disposal of sewage sludge, providing a long-term outlet for this troublesome waste stream.
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