Microbial activities in soil amended with sewage sludges

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Abstract
Microbial populations and enzyme activities in a Light-colored Andosol amended with sewage sludges were studied in relation to the decomposition of the sludges. Six kinds of sewage sludges were mixed with the soil at rates of 5 and 1%. Subsequently, the mineralization of C and N, numbers of soil microorganisms, and activities of soil enzymes were determined during an 8-week incubation period at 28°C. The mineralization rates of organic C and N increased rapidly and reached maximum values within the first 3 days, but then decreased rapidly. Similar trends were observed for the number of bacteria and proteinase activity in the soil, which showed high correlation coefficients with the mineralization rates of C and N. The numbers of actinomycetes and fungi reached maximum values after 2 or 3 weeks of incubation and remained at the same levels thereafter. The amount of amino acid-N in 6 N HCI extracts of the soil amended with the sludges decreased markedly during the incubation period. These results suggest that proteinase-producing bacteria contribute significantly to the rapid decomposition of sewage sludge in the early days after the application and that actinomycetes and fungi contribute to the gradual decomposition after the end of the rapid decomposition.

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