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

Methods for measuring soil microbial biomass C were reviewed. The basic ideas behind the fumigation-incubation method, the fumigation-extraction method, the substrate-induced respiration method, and the ATP method were examined together with the advantages, disadvantages, and limitations as reported in the literature and those found by our own recent investigations. The fumigation - incubation method is the basic technique which is also used for calibration of the three other methods. It is characterized by simple performance without the need of expensive 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 limitations are not considered, too low or even negative biomass values will be obtained. These restrictions are largely overcome by the fumigation-extraction method. However, the k(EC) factor applied to calculate microbial biomass C from the C additionally made extractable by the fumigation is still controversial. The substrate-induced respiration requires expensive equipment for the hourly measurement of soil respiration. This method is also susceptible to amendment of soils with C sources, leading to an overestimate of biomass C. Although a few authors disagree with some basic assumptions behind the methods described, they 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 for the extraction and measurement of ATP has led to biomass C:ATP ratios which vary between about 150 and 1000. Our own current investigations are expected to shed more light on the problems of ATP extraction. Preliminary results indicate that a constant biomass C:ATP ratio of about 200 may be more realistic.