CURRENT METHODS FOR MEASURING MICROBIAL BIOMASS-C IN SOIL - POTENTIALS AND LIMITATIONS

Methods for measuring soil microbial biomass C were reviewed. The basi c ideas behind the fumigation-incubation method, the fumigation-extrac tion method, the substrate-induced respiration method, and the ATP met hod were examined together with the advantages, disadvantages, and lim itations as reported in the literature and those found by our own rece nt investigations. The fumigation - incubation method is the basic tec hnique which is also used for calibration of the three other methods. It is characterized by simple performance without the need of expensiv e equipment. Its application is limited to soils with a pH above 5 and to soils that do not contain easily degradable C sources. If these li mitations are not considered, too low or even negative biomass values will be obtained. These restrictions are largely overcome by the fumig ation-extraction method. However, the k(EC) factor applied to calculat e microbial biomass C from the C additionally made extractable by the fumigation is still controversial. The substrate-induced respiration r equires expensive equipment for the hourly measurement of soil respira tion. This method is also susceptible to amendment of soils with C sou rces, leading to an overestimate of biomass C. Although a few authors disagree with some basic assumptions behind the methods described, the y are widely used and accepted. The use of ATP to measure biomass C in soil is far more uncertain. A high diversity of applied techniques fo r the extraction and measurement of ATP has led to biomass C:ATP ratio s which vary between about 150 and 1000. Our own current investigation s are expected to shed more light on the problems of ATP extraction. P reliminary results indicate that a constant biomass C:ATP ratio of abo ut 200 may be more realistic.