Comparison of microbial properties measured by O2 consumption and microcalorimetry as bioindicators in forest soils

Microbial biomass C was determined in surface organic horizons and in the top 15 cm of the mineral soil of five European coniferous forests. Both oxygen-consumption and heat production following glucose addition were used to measure microbial biomass C. The undisturbed soils showed a marked gradient down the profile with regard to organic C and total N content and microbial biomass. Average values of microbial biomass decreased from the fresh fallen litter in the L horizon (range in five soils) to the B horizon in all five mineral soils from 6667 to 227 μg g−1 DW (O2-consumption) and from 7555 to 206 μg g−1 DW (heat production), respectively. Correlation analysis between results of O2-consumption and heat production was carried out using the data from the analysis of complete soil profiles with that from single horizons. The results of both methods for measuring microbial biomass C were, in general, highly correlated (P<0.01). Significant correlations were also found between the results of both methods for measuring microbial activity. The correlations between soil chemical properties (pHCaCl2, organic C, total N and C-to-N ratio) and microbial biomass C indicated very similar relationships for both methods. There were significant relationships (P<0.05) of microbial biomass C to Ph in F- and H-layers, and in the mineral horizons of microbial biomass to pH, soil organic C and total N. The specific activity of microbial biomass was calculated as the metabolic quotient (μg O2 mg−1 Cmic h−1 [Cmic detected by O2-consumption]) and the caloric quotient (μW mg−1 Cmic [Cmic detected by heat production]). Specific activity showed different patterns with depth and between soils. Metabolic quotient and caloric quotient were poorly correlated and indicate the limitations of specific activity as a bioindicator in forest soils.