THE VARIABILITY BETWEEN DIFFERENT ANALYTICAL PROCEDURES AND LABORATORIES FOR MEASURING SOIL MICROBIAL BIOMASS C AND BIOMASS N BY THE FUMIGATION EXTRACTION METHOD

The interlaboratory variations in the fumigation extraction method and the analytical procedures for measuring C and N in the soil microbial biomass were tested with one soil sample, and two soil extracts (non- fumigated and fumigated) sent to 25 different laboratories. Four group s of analytical procedures for organic C, i.e. (1) oven oxidation/IR d etection, (2) UV-persulfate oxidation/IR detection, (3) UV-persulfate oxidation/colorimetric detection and (4) dichromate oxidation/titratio n, and three groups for total N, i.e. (1) Kjedahl reduction to NH4+, ( 2) UV-persulfate and (3) persulfate-borate oxidation to NO3- were used by the different laboratories. The coefficient of variation for C and N measurements between different laboratories and analytical procedur es varied between 15 and 34% in non-fumigated samples, between 13 and 20% in fumigated samples, and between 12 and 24% in the differences E- C and E-N. The average coefficients of variation between the replicate measurements within one laboratory were much smaller, i.e. they varie d between 3.0 and 9.2% in non-fumigated samples, between 2.4 and 5.5% in fumigated samples, and between 4.5 and 12.8% in the differences E-C and E-N. Extraction and fumigation were not the major source of the v ariations observed. They were mainly a result of differences in the an alytical procedures used to measure the low concentrations of C and N in the extracts. However, all of these analytical procedures should be able to measure correct values if they are properly calibrated and pe rformed.