ACTIVITY AND BIOMASS OF SOIL-MICROORGANISMS AT DIFFERENT DEPTHS

We measured microbial biomass C and soil organic C in soils from one g rassland and two arable sites at depths of between 0 and 90 cm. The mi crobial biomass C content decreased from a maximum of 1147 (0-10 cm la yer) to 24 mu g g(-1) soil (70-90 cm layer) at the grassland site, fro m 178 (acidic site) and 264 mu g g(-1) soil (neutral site) at 10-20 cm to values of between 13 and 12 mu g g(-1) soil (70-90 cm layer) at th e two arable sites. No significant depth gradient was observed within the plough layer (0-30 cm depth) for biomass C and soil organic C cont ents. In general, the microbial biomass C to soil organic C ratio decr eased with depth from a maximum of between 1.4 and 2.6% to a minimum o f between 0.5 and 0.7% at 70-90 cm in the three soils. Over a 24-week incubation period at 25 degrees C, we examined the survival of microbi al biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the cont ents of microbial biomass C at 0-30 cm were significantly lower than t he initial values. At depths of between 30 and 90 cm, the microbial bi omass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5. 8% of soil organic C was mineralized at 0-30 cm in the three soils and 4.8% at 30-90 cm. Generally, the metabolic quotient qCO(2) values inc reased with depth and were especially large at 70-90 cm in depth.