Influence of soil acidity on depth gradients of microbial biomass in beech forest soils

The objectives of the study were to investigate mineral soil profiles as a living space for microbial decomposers and the relation of microbial properties to soil acidity. We estimated microbial biomass C on concentration (mu g g(-1) DW) as well as on volume basis ( g m(-2)) and the microbial biomass C to soil organic C ratio along a vertical gradient from L horizon to 20 cm in the mineral soil and along a gradient of increasing acidity at five beech forest stands in Germany. Microbial biomass C concentration ranged from 17,000 - 34,000 mu g C-mic g(-1) DW in the litter layer and decreased dramatically down the profile to 29 - 264 mu g C-mic g(-1) DW at 15 - 20 cm depth in the mineral soil. This represents depth gradients of microbial biomass C concentrations ranging from a factor of 65 in slightly acidic and up to 875 in acidic soils. In contrast, microbial biomass C calculated on a volume basis ( g C-mic m(-2)) showed a different pattern since a considerable part of the microbial biomass C was located in the mineral soils. In the soil profile 22-34% of the microbial biomass C was found in the mineral soil at strictly acidic sites and as much as 64 - 88% in slightly acidic soils. The microbial biomass C to soil organic carbon ratios decreased in general down from the L horizon in the forest floor to 0 - 5 cm depth in the mineral soils. In strongly acidic mineral soils however, the C to soil organic carbon ratio increased with depth, suggesting a positive relation to increasing pH. We conclude from depth gradients of soil pH and microbial biomass C to soil organic carbon ratio that pH affects this ratio at acidic sites. The inter-site comparison indicates that acidity restricts microbial biomass C in the mineral soils.