MECHANISMS FOR CARBON AND NUTRIENT RELEASE AND RETENTION IN BEECH FOREST GAPS .2. THE ROLE OF SOIL MICROBIAL BIOMASS

Field levels of soil microbial biomass C and N, determined by the chlo roform fumigation-extraction method, showed no marked seasonal pattern in limed and unlimed gaps and the surrounding beech stand; average co ntents were 530, 532 and 674 kg microbial C ha(-1); and 65, 68 and 87 kg microbial N ha(-1) respectively. Liming increased microbial. biomas s in the stand but had no influence on the size of the microbial pool in the gap. The decrease in microbial biomass in gaps was attributed t o a decline of ectomycorrhizal hyphae. Due to the absence of available carbon soil microbial biomass was not a significant nutrient sink fol lowing forest disturbance. In situ N mineralization at these sites cou ld not be explained by fluctuations in microbial N. Laboratory measure ments of CO2 evolution from soil samples, which were in good agreement with field CO2 measurements, indicated a decline in easily decomposab le soil organic matter during the growing season. High metabolic quoti ents (mu g CO2-C mg C-mic, (-1) d(-1)) of the microflora in gaps indic ated inefficient energy use.