Evidence for the use of a control in the fumigation-incubation method for measuring microbial biomass carbon in soil

Soil microbial biomass C (Bc) is calculated from Bc = Fckc in the original fumigation-incubation (FI) method. In this expression, kc is a proportionality constant to allow for the fact that not all the C in the microorganisms killed by the fumigant (CHCl3) is evolved as CO2 during the specified 10 day incubation. The flush (Fc) is taken as the CO2 evolved in 10 days by the fumigated soil, less that evolved in 10 days by unfumigated soil — the ‘control’. Voroney and Paul (1984) argued that this unfumigated control should not be deducted, thus obtaining estimates of microbial biomass C that were much larger, often by a factor of 2 or more. At present some investigators use a control and some do not. This paper describes an experiment designed to resolve this issue. A grassland soil was incubated with uniformly 14C-labelled glucose for 27 days, then fumigated with CHCl3. Labelled and unlabelled organic C extractable to 0.5 m K2SO4 were measured in the fumigated and unfumigated soils at this time and used to calculate Ec, the amount of biomass C made extractable to K2SO4 by CHCl3. Evolution of 14C-labelled and unlabelled CO2 was also measured over the next 30 days in the fumigated and unfumigated soils. All the results were entirely consistent with the use of a control as proposed in the original method. Once the flush of decomposition had subsided, the CO2 respiration curves of fumigated and unfumigated soil were nearly parallel. Following fumigation, the specific activity of the CO2 evolved from the fumigated soil rapidly increased and then returned to a level close to that in the unfumigated soil, suggesting that the same pool of organic matter was being mineralised in both fumigated and unfumigated soils, once the heavily-labelled biomass killed by the fumigant had decomposed. Other evidence was that the specific activity of Fc (139 Bq mg−1 C) did not differ significantly from that of Ec (136 Bq mg−1 C), both calculated using the appropriate control.