Soil Microbial Biomass Determination

Basic Idea: The quantity of microbial biomass in a given soil sample is inherently difficult to measure. To some degree, any measurement of microbial biomass is relative – different methodologies or variations in methodologies will yield microbial biomass estimates that are not directly comparable. For this reason, if you want to measure microbial biomass – choose a method and stick with it exactly. Here, I’ll discuss 2 methods to measure microbial biomass: substrate induced respiration (SIR) and simultaneous chloroform fumigation-extraction (sCFE), a modification of the traditional chlorform-fumigation extraction method. SIR estimates microbial biomass based on the short-term respiration rates (CO2 production) when soil is given an excess supply of labile C. The higher the respiration rate, the more microbial biomass in a given soil. The SIR method is fairly straightforward, consistent, and reasonably quick, however, the biomass estimates are relative – the method will not give direct estimates of microbial biomass C (or N) in soil. sCFE measures microbial biomass based on the amount of microbial C or N extractable from soil after treatment with chloroform, a cell membrane disruptor. There is a lot of confusion about general chloroform fumigation-extraction techniques, and, (in my opinion) the technique is often carried out in a manner that makes cross-comparison of biomass estimates difficult. For these reasons, I have included a lengthy treatise on the technique (see pages 2 – 4) along with a full description of my modifications to the traditional chloroform-fumigation extraction method. The sCFE technique combines the soil extraction and chloroform exposure steps, increasing the simplicity and efficacy of the normal fumigation-extraction method. The modified method yields estimates of microbial C and N that are well correlated with those obtained by the normal fumigationextraction method across a range of soil types. The simultaneous chloroform slurryextraction method is particularly useful for studying soils with a range of porosities and/or water contents where differences in chloroform diffusion rates through soils may affect biomass extraction efficiencies when using the normal fumigation-extraction method.