Effects of sewage sludge pre-treatment on microbial biomass and biovailability of heavy metals

The amounts of DTPA-extractable metals, soil organic C and microbial biomass C were measured in a silty loam soil near Ravenna (Italy), where dehydrated, or dehydrated and composted sewage sludge had been applied once a year at a rate of 7.5 and 15 t ha(-1) from 1989. Our purpose was to study the relationship between DTPA-extractable metals and soil organic matter content, microbial biomass and soil total metal content. Five years later, microbial biomass C did not differentiate the 7.5 t ha(-1) treatment from control, while it increased (+ 27%) when the rate of 15 t ha(-1) was added. Soil organic C, on the contrary, was not significantly different among the treatments. The total content of Zn, Cu, Ni, and Pb in the soil treated with sewage sludge did not differ from that of control soil. DTPA extracted at least 6% of total Zn, 3% of total Ni, 25% of total Cu, and 23% of total Pb, but only DTPA-extractable Zn and Ni increased from control soil to 7.5 and 15 t ha(-1) sewage sludge treatments. DTPA-extractable Zn was correlated with total Zn content and, along with DTPA-extractable Ni, was correlated with microbial biomass C. These relationships suggest that the bioavailability of Zn and Ni in the soil are modulated to some extent by microbial biomass. DTPA-extractable Cu and Ph were highly correlated only with their total soil content. (C) 1998 Elsevier Science B.V. All rights reserved.