Enzyme activities and heavy metals concentration in soil amended with sewage sludge

Municipal sewage sludge (MSS) and yard waste compost (YWC) provide amendments useful for improving soil structure and nutrient status. However, soil amendments contain heavy metals that may potentially affect soil microbes and the enzymes they produce. A field study was conducted using three soil managemet practices (MSS, YWC, and native soil). Broccoli (Brassica oleracea L.) seedlings were planted, and the activities of the enzymes hydrolyzing urea (urease), sucrose (invertase), and p-nitrophenyl phosphate (acid and alkaline phosphatase) were determined in spring and fall agricultural soil. The greater soil urease and invertase activities in spring soil amended with MSS provided evidence of increased soil microbial population. On the contrary, the application of YWC in spring did not alter soil urease or invertase activities to any appreciable extent. Overall acid and alkaline phosphatase were stimulated in soil amended with YWC. Nickel, Zn, and Cu increased in soil amended with MSS while, concentration of Pb increased after addition of YWC to native soil. Nickel and Pb were taken up by broccoli plants grown in MSS amended soil, but their concentration in broccoli heads were below the Codex Commission Allowable Limits.