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 and 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 sludge amendment rate caused an increase in both pH and electrical conductivity (EC). However, pH decreased while EC increased and then decreased along the incubation time. Nevertheless salinity and heavy metal contents of the soil sludge mixture were all within the safety guidelines. Soluble NH₄⁺, NO₃^2- and PO₃^2- increased after amending the soil with 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 was the same in all treatments that CO₂ evolution increased initially and then decreased till the end of the incubation period. All the treatments had peak CO₂ evolution at day 7, except for the soil amended with 350 g kg^-1 of sludge which had peak CO₂ evolution at day 2. Similarly, the percentage of C-mineralization decreased with an increase in sludge amendment rate. The present experiment indicated that an application rate of 50-150 g kg^-1 sludge for sandy soil would have the optimal beneficial effect on the soil in terms of microbial activity and nutrient transformation.