An extraction method for measuring soil microbial biomass C

Abstract

The effects of fumigation on organic C extractable by 0.5 M K 2SO 4 were examined in a contrasting range of soils. E C (the difference between organic C extracted by 0.5 M K 2SO 4 from fumigated and non-fumigated soil) was about 70% of F C (the flush of CO 2-C caused by fumigation during a 10 day incubation), meaned for ten soils. There was a close relationship between microbial biomass C, measured by fumigation-incubation (from the relationship Biomass C = F C /0.45) and E C given by the equation: Biomass C = (2.64 ± 0.060) E C that accounted for 99.2% of the variance in the data. This relationship held over a wide range of soil pH (3.9–8.0). ATP and microbial biomass N concentrations were measured in four of the soils. The (ATP) (E C) ratios were very similar in the four soils, suggesting that both ATP and the organic C rendered decomposable by CHCl 3 came from the soil microbial biomass. The C:N ratio of the biomass in a strongly acid (pH 4.2) soil was greater (9.4) than in the three less-acid soils (mean C:N ratio 5.1). We propose that the organic C rendered extractable to 0.5 m K 2SO 4 after a 24 h CHCl 3-fumigation ( E C ) comes from the cells of the microbial biomass and can be used to estimate soil microbial biomass C in both neutral and acid soils.