EFFECT OF SEWAGE-SLUDGE AMENDMENT ON SOIL MICROBIAL ACTIVITY AND NUTRIENT MINERALIZATION

An incubation experiment was performed to, study the effect of sewage sludge on microbial respiration and nutrient mineralization in a sandy soil as an indication of its effects on soil biological properties an d nutrient transformation. Sewage sludge was amended with a sandy soil at 0, 25, 50, 150 and 350 g kg(-1) fresh weight. An increase in the s ludge amendment rate caused an increase in both pH and electrical cond uctivity (EC). However, pH decreased while EC increased and then decre ased along the incubation time. Nevertheless salinity and heavy metal contents of the soil sludge mixture were all within the safety guideli nes. Soluble NH4+, NO32- and PO32- increased after amending the soil w ith sewage sludge, but increasing the application rate to 350 g kg(-1) of sludge decreased the N and P mineralization efficiency and created an adverse effect on nitrification. The daily CO2 evolution pattern w as the same in all treatments that CO2 evolution increased initially a nd then decreased till the end of the incubation period. All the treat ments had peak CO2 evolution at day 7, except for the soil amended wit h 350 g kg(-1) of sludge which had peak CO2 evolution at day 2. Simila rly, the percentage of C-mineralization decreased with an increase in sludge amendment rate. The present experiment indicated that an applic ation rate of 50-150 g kg(-1) sludge for sandy soil would have the opt imal beneficial effect on the soil in terms of microbial activity and nutrient transformation. (C)1998 Elsevier Science Ltd.