Effect of sewage sludge on microbial biomass, basal respiration, metabolic quotient and soil enzymatic activity

Monitoring soil quality by means of biological indices can be of help for the management and sustainability of soils that received sewage sludge application. The objective of this study was to evaluate the effect of long-term and continued application of sewage sludge rates on microbial biomass, basal respiration, metabolic quotient and enzymatic activity of a Dark Red Dystroferric Latosol under field conditions in the tropical region. To that effect, four applications of sewage sludge from an industrialized region were performed on a Dark Red Dystroferric Latosol. Sludge was incorporated to the soil at concentrations of 0, 1, 2, 4 and 8 times the recommended rate based on their nitrogen content in four consecutive corn cultivations. The results showed that basal respiration, the C and N microbial biomass, metabolic quotient (qCO2) and enzymatic activity in the soil increased as sewage sludge was added, and their values were positively correlated with sewage sludge doses. The activities of soil urease and amylase increased as sludge doses increased and were significantly correlated with soil microbial biomass. These results suggest that the amount of sewage sludge applied has to be calculated based on the N crop needs, and annual applications must be avoided to prevent over-applications.