Submitted to: Methods of Soil Analysis
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2006
Publication Date: 8/3/2007
Citation: Bailey, V.L., Smith, J.L., Bolton, H.J. 2007. SUBSTRATE-INDUCED RESPIRATION AND SELECTIVE INHIBITION AS MEASURES OF MICROBIAL BIOMASS IN SOILS. pp 515-526. Soil Sampling and Methods of Analysis. Carter, M.R. and Gregorich, E.G., eds. Canadian Society of Soil Science. Ottawa, Canada. Interpretive Summary: Microorganisms (microbial biomass) in soil are responsible for the decomposition of organic matter and the cycling of nutrients required by plants. To fully understand nutrient cycling in soil systems it is useful to know the magnitude of the soil organism population. Thus a clear concise method to determine the soil organism population needs to be developed. We explain and outline 2 methods, the substrate –induced respiration method (SIR), and the selective inhibition method, for determining the magnitude of the soil organisms and also the ratio of soil bacteria to soil fungi. We discuss the pro’s and con’s of each method so that scientists can determine if these procedures are appropriate to their experiments. The benefit of this document is to scientists who study the rates of nutrient cycling in soils or the rate of decomposition of organic compounds in soil including toxic chemicals in the environment.
Technical Abstract: Microbial biomass can be extremely valuable for process studies because of the time dependence of microbially-mediated reactions. The mineralization of elements such as carbon and nitrogen and the degradation of many (organic) chemicals have kinetics that depend upon the concentration of substrate and which require microbial biomass as a reactive agent. If rapid methods for biomass determinations were reliable and were not influenced by changing soil conditions, the effect of microbial biomass on the kinetics of mineralization, chemical degradation and nutrient uptake could be determined. The substrate –induced respiration method (SIR) is a simple, rapid and economical method to determine microbial biomass C in soils and residues. The theory behind the method is that the initial response of microorganisms to a soluble energy-yielding substrate would be proportional to the mass of organisms. The selective inhibition method uses fungicides and bactericides to measure the ratio of fungal activity to bacterial activity. This approach pre-dates the calibration of substrate-induced respiration as a measure of microbial biomass, but is essentially a variation of substrate-induced respiration. In this paper we describe each of these methods.