INOCULUM IN THE FUMIGATION METHOD FOR SOIL BIOMASS DETERMINATION

The size of the microbial population used to inoculate six fumigated soils of pH 3.2 to 6.0 was shown to affect the respiration during the next 10 days. This is principally ascribed to the reduced decomposition of organic matter, other than fumigated microbial cells, in the absence of a fully competent microflora. A small inoculum of 0.4% (w/w) was better than no inoculation, but an inoculum of at least 10% was required to achieve maximal respiration. It is proposed that soil biomass C determinations should be made with a 10% inoculum in the fumigated soil and subtracting the total CO2-C release from an unfumigated control soil over 10 days from the value for the fumigated soil during the same period. For acid soils (pH < 4.5) a 20% inoculum should be used. A correction should be applied for the reduced proportion of fumigated soil.