FUMIGATION EXTRACTION METHOD TO ESTIMATE MICROBIAL BIOMASS IN HEATED SOILS

A fumigation-extraction method that measures the fraction of the biomass rendered extractable to K2SO4 after microbial cells lysis by chloroform was used to estimate microbial biomass C and N in forest soil samples unheated or heated at 160, 350 or 600-degrees-C for 30 min, 2 or 4 h. The biomass C and N of the unheated samples were 1011-mu-g C g-1 and 124-mu-g N g-1 dry soil. Chloroform fumigation of the heated samples did not solubilize non-microbial soil organic matter. The microbial C and N in the heated samples were reduced to undetectable amounts regardless of the temperature and duration of heating. After restoring the microbial population in the heated soils by inoculation with fresh unheated soil and incubation, microbial biomass C and N were detected in the samples heated at 160-degrees-C (441-mu-g C g-1 and 42-mu-g N g-1 dry soil) and 350-degrees-C (63-mu-g C g-1 and 11-mu-g N g-1 dry soil) whereas the values in samples heated at 600-degrees-C were below the limits of detection. The decrease in biomass values were in accordance with a decrease in the number of microorganisms and hyphal length the recovery of which was inversely related to the temperature of heating. The results suggest that the fumigation-extraction technique can be used to estimate microbial biomass C and N in heated soils.