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This study aimed to identify how the ratio of inorganic-to-organic components in animal manure compost (AMC) affected both lead immobilization and microbial activity in lead-contaminated soil. When AMC containing 50% or more inorganic fraction with high phosphorous content was applied to contaminated soil, the amounts of water-soluble lead in it were suppressed by over 88% from the values in the soil without compost. The residual fraction under sequential extraction increased with the inorganic fraction in the AMC; however, in those AMCs, the levels of microbial enzyme activity were the same or less than those in the control soil. The application of AMC containing 25% inorganic fraction could alter the lead phases to be more insoluble while improving microbial enzyme activities; however, no suppression of the level of water-soluble lead existed during the first 30 days. These results indicate that compost containing an inorganic component of 50% or more with high phosphorus content is suitable for immobilizing lead; however, in the case where low precipitation is expected for a month, AMC containing 25% inorganic component could be used to both immobilize lead and restore microbial activity.

In the present study, we aimed to understand how amelioration of animal manure compost (AMC) with high phosphorus and low water-soluble organic carbon (WSOC) contents can simultaneously immobilize lead and reduce lead mobility and bioavailability in soil irrespective of the animal source. The amount of water-soluble lead in the soil amended with swine compost (SC) was not suppressed as compared with that in the soil without compost, whereas it was suppressed in the case of the soil amended with cattle compost (CC). The lead phases in the soil amended with SC became less soluble; however, those in the soil amended with CC were equivalent to those in the soil without compost. The ameliorated cattle and SCs with high phosphorus and low WSOC contents simultaneously induced a significant reduction in the concentration of water-soluble lead and ensured the formation of higher concentrations of insoluble lead phases. The microbial enzyme activities in the soil amended with the ameliorated compost were lower than those in the soil amended with the SC. This study suggests that ameliorated AMC can alter lead phases to insoluble forms and suppress the level of water-soluble lead, simultaneously. Therefore, such ameliorated AMC with high phosphorus and low WSOC contents would be suitable as a lead immobilization material.

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