Freeze-dried soil extraction method for the measurement of microbial biomass C

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 (C_{K2 SO4} or C_NaHCO3). The net freeze-dry stimulated increase in extracted C was correlated (r ²=0.98 for C_{K2 SO4} or 0.93 for <$>rm C_{NaHCO_3})<$> more closely with microbial biomass C (C_MB) measured as net evolution of CO₂–C by chloroform fumigation incubation (CFI) than with total C (r ²=0.42 for C_{K2 SO4} or 0.47 for C_NaHCO3). Based on linear regression equations, extraction efficiency coefficients (K _EC) were used to calculate C_MB from C_{K2 SO4} or C_NaHCO3 as follows: C_MB=C_{K2 SO4}/0.152±0.004 C_MB=C_NaHCO3/0.257±0.01 The relationship between the C_MB and the flushes of C extracted after rehydration of freeze-dried soil showed that the K _EC values were more consistent for C_{K2 SO4} than C_NaHCO3. The freeze-dried soil extraction was a fast, precise, reliable and safe method for measuring microbial biomass C in soil. Received: 27 May 1996