Abstract
A new method for the measurement of microbial biomass C by direct extraction of freeze-dried soil with either 0.5M K2SO4 or 0.5M NaHCO3 was evaluated. The underlying principle of the method is that rehydrating a freeze-dried soil releases cytoplasmic organic compounds from desiccated and disrupted microbial cells. Nineteen soils under various management regimes were sampled to test the proposed method, in which each soil sample was split into two subsamples. One subsample was kept in the field-moist condition at 4°C. The other subsample was brought to 100% water-holding capacity and frozen at –20°C for 24h. The frozen soil was then freezedried. Both subsamples were extracted with 0.5M K2SO4 or 0.5M NaHCO3 at a soil-to-extractant ratio of 1-to-4 (w/v) and organic C determined in the extract (CK2SO4 or CNaHCO3). The net freeze-dry stimulated increase in extracted C was correlated (r2=0.98 for CK2SO4 or 0.93 for \rm C_{NaHCO_3}) more closely with microbial biomass C (CMB) measured as net evolution of CO2–C by chloroform fumigation incubation (CFI) than with total C (r2=0.42 for CK2SO4 or 0.47 for CNaHCO3). Based on linear regression equations, extraction efficiency coefficients (KEC) were used to calculate CMB from CK2SO4 or CNaHCO3 as follows:
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