EVALUATION OF THE FUMIGATION-EXTRACTION METHOD FOR THE DETERMINATION OF MICROBIAL C AND N IN A RANGE OF FOREST SOILS

The release of extractable C and N by chloroform fumigation of 10 forest and two arable soils was compared with microbial C measured by direct microscopic counting (membrane filter technique). The soils varied with respect of pH (3.9-6.8) and content of organic C (2.6-36%). The amount of microbial C based on microscopic counts ranged from 332 to 1342 μ g cm-3. The release of C caused by fumigation correlated well with the results obtained from direct microscopic counting (R2 = 0.87). The regression model calculated for microbial C is Cm = (1.30 Cf + 309) μg cm-3 soil, where subscript m refers to the values for microscopic counting and subscript f to the release caused by fumigation. Assuming that fumigation causes a release of microbial N in the same proportion as for microbial C, the following regression model for microbial N was obtained: Nm (1.38 Nf + 45.3) fig cm-3 soil. The average amount of microbial C determined with the fumigation-extraction method for coniferous forest soils, deciduous forest soils and arable soils was 1.19, 1.13 and 1.40% of total C, respectively. The avarage amount of microbial N in these soils was 5.9, 3.4 and 2.5% of total N. The results suggest that the fumigation-extraction method can be used for the determination of microbial C and N in different types of forest soil. © 1990.