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 m atter calcareous soil (calcixerollic xerochrept, Bellaterra, Spain), a t concentrations of 7.5% and 15%. Control soil (0% sludge), sludge, an d mixtures were incubated for 1 year inside a chamber in a 15-L contai ner. Moisture and temperature were controlled. Microbial biomass and C O2 production were measured. Soil respiration potential was proportion al to soil sewage sludge content. No microbial growth inhibition was d etected in soil-sewage sludge mixtures despite the fact that the 15% s ewage sludge concentration doubled the recommended maximum input appli cable to the soil. Sewage sludge incubated without soil skewed lower r espiration values. One year after sewage application, respiration pote ntial of soil-sewage sludge mixtures and sewage sludge dropped to lowe r values, suggesting stabilization of organic matter. Mineralization c oefficients were high at the beginning of the incubation period, in co mparison with other calcareous soils, but decreased 12 months later an d reached values similar to those of the control soil. This suggests a rapid stabilization of organic matter from the sludge. Microbial biom ass was also proportional to sewage contents in soil. The microbial me tabolic quotient (qCO(2)) was low in control soil, suggesting a very h umified organic matter. Sewage sludge had a very high qCO(2) and indic ated high activity per microbial unit. Both 7.5 and 15% sewage sludge treatments showed similar high quotients, indicating good conditions f or sludge degradation by microorganisms.