ESTIMATION OF SOIL MICROBIAL C BY A FUMIGATION EXTRACTION METHOD - INFLUENCE OF SEASONS, SOILS AND CALIBRATION WITH THE FUMIGATION INCUBATION PROCEDURE

Extractable-C in soils from perennial grasslands was measured by an extraction method using 0.5 m K2SO4. In fresh, unfumigated samples taken at different seasons, extractable-C contents were related positively to the field-moisture content of each soil. In contrast, values of extractable-C flush, viz. the difference between the amounts of C extracted from chloroform-fumigated and unfumigated soil, were generally highest in the driest, summer or autumn samples. Extractable-C flush was converted to microbial biomass C by calibration with the fumigation-incubation procedure, to obtain kEC factors. Various modifications of the incubation procedure were investigated, and had a marked effect on kEC values; the use of a small inoculum and fumigated control appeared the most satisfactory procedure for calculating microbial C, for fresh and pre-incubated samples. The mean kEC value of seven soils was then ca 0.33. with a SD of 0.07. The kEC values of individual soils varied with season of sampling, and were generally highest in autumn. Variations of kEC values amongst the different soils were. however, appreciably greater. Some limitations of the fumigation-extraction method and of the use of a common kEC-factor for different soils are discussed. Preliminary calibration of extractable-C flush values, for conversion to microbial biomass C content, is recommended for detailed studies of an individual soil. © 1990.