MICROBIAL BIOMASS MEASUREMENTS IN FOREST SOILS - DETERMINATION OF KC VALUES AND TESTS OF HYPOTHESES TO EXPLAIN THE FAILURE OF THE CHLOROFORM FUMIGATION INCUBATION METHOD IN ACID SOILS

The chloroform fumigation-incubation method underestimates the amount of microbial biomass C in strongly acid soils (pH < 4.5). Several explanations for the failure of the method were examined using 10 forest soils that ranged in pH from 3.4 to 7.2. The hypothesis that chloroform might form compounds toxic to microbial recolonizers in strongly acid soils, but not in soils of higher pH, was tested using an alternative fumigant, carbon disulphide. CO2-C evolution was very similar with both fumigants and this explanation was rejected. Another possibility is that the proportion of microbial C evolved as CO2-C from the decomposition of microbial cells killed by fumigation (kc) is less in strongly acid soils than in soils of higher pH, so that low values for biomass will be obtained if the value of kC commonly used for near-neutral soils (0.45) is used on strongly acid soils. This was tested by measuring the 14CO2 evolved when 14C-labelled fungi and bacteria were added to the soils, which were then fumigated and incubated. The values of kC thus obtained were indeed less in strongly acid soils; the mean kC for soils below pH 4.5 was 0.30, compared to a mean of 0.46 in soils above pH 4.5. In the fumigation-incubation method, the fumigated soil is inoculated with a few mg of non-fumigated soil after the fumigant is removed but before incubation begins, to increase the size of the microbial recolonizing population. In soils above about pH 5, it makes little difference whether or not the soil is inoculated. However, in some of our strongly acid soils this was not so and a large inoculum increased the quantity of CO2 produced after fumigation. Neither the use of an inappropriate kC factor nor inadequate inoculation appear, in themselves, to be sufficient to explain why the fumigation-incubation method underestimates the amount of microbial biomass in strongly acid soils. We suggest the main reason is that the recolonzing population in strongly acid soils is unable to metabolize non-microbial organic matter as fast as the native population in non-fumigated soils, so that the use of a non-fumigated soil as control will lead to low values for biomass.