RESPIRATION POTENTIAL OF MICROBIAL BIOMASS IN A CALCAREOUS SOIL TREATED WITH SEWAGE-SLUDGE

Urban sewage sludge from Gerona (Spain) was applied To a low organic matter calcareous soil (calcixerollic xerochrept, Bellaterra, Spain), at concentrations of 7.5% and 15%. Control soil (0% sludge), sludge, and mixtures were incubated for 1 year inside a chamber in a 15-L container. Moisture and temperature were controlled. Microbial biomass and CO2 production were measured. Soil respiration potential was proportional to soil sewage sludge content. No microbial growth inhibition was detected in soil-sewage sludge mixtures despite the fact that the 15% sewage sludge concentration doubled the recommended maximum input applicable to the soil. Sewage sludge incubated without soil skewed lower respiration values. One year after sewage application, respiration potential of soil-sewage sludge mixtures and sewage sludge dropped to lower values, suggesting stabilization of organic matter. Mineralization coefficients were high at the beginning of the incubation period, in comparison with other calcareous soils, but decreased 12 months later and reached values similar to those of the control soil. This suggests a rapid stabilization of organic matter from the sludge. Microbial biomass was also proportional to sewage contents in soil. The microbial metabolic quotient (qCO(2)) was low in control soil, suggesting a very humified organic matter. Sewage sludge had a very high qCO(2) and indicated high activity per microbial unit. Both 7.5 and 15% sewage sludge treatments showed similar high quotients, indicating good conditions for sludge degradation by microorganisms.