Background, Aims and Scope. Soil microbial community diversity has been suggested as a way of assessing the 'health' or 'quality' of soils. While molecular (genetic) or biochemical (phenotypic) measurements of microbial diversity have their place, functional diversity is popular in that it relates to the activity of the soil microflora, particularly in the carbon cycle. Community Level Physiological Profiles (CLPPs) are usually assessed by carbon substrate utilization. The aim here is to review the various methods available for CLPP assessment.
Main Features. The original method for CLPP determination was based upon the Biolog plate with its range of 95 carbon substrates and many studies have been employing this over the past 15 years. However, a number of criticisms have been leveled at the method, principally that it relies upon the growth of an extracted microbial population, which may not represent the true functioning of the whole soil. A multiple carbon-source, substrate induced respiration method (multi-SIR) was developed that did measure the response of the whole soil community without the need for growth but it did not have the convenience of the microtitre plate format. MicroReSp (TM) was designed to overcome the deficiencies in both methods and we have applied it to a range of media including mineral and organic soils, sediments and litter. In addition we have adapted the method to the testing of a wide range of carbon substrates such as hydrocarbons, terpenes and pesticides.
Results. The MicroReSp (TM) technique covers the middle road of relevance and convenience, being a 'whole soil' method in a flexible microtitre plate format. It also has the advantage that it can be adapted for the use of radiolabelled (C-14) substrates, which increases the specificity and sensitivity of the assay. A few direct comparisons between MicroReSp (TM) have been made. We have found that its discriminatory ability compares favourably with Biolog while other users have reported it to be superior to the multiple SIR approach. A number of laboratories around the world are now utilizing MicroReSp (TM) and reporting good sensitivity to changes to the microbial community due to such varied factors as age of forest rehabilitation, heavy metal treatment, hydrocarbon exposure, salinity, peatland vegetation, cropping system and tree clone type.
Discussion. A comparison of the available methods reveals that each has its advantages and drawbacks. The choice of method may depend upon the particular hypotheses or questions.
Conclusions. MicroReSp (TM) offers a convenient, rapid and sensitive method for the determination of Community Level Physiological Profiles. Its application in a number of case studies has demonstrated its utility and advantages over other methods though its full potential for characterizing soil activity is yet to be realized. Perspectives. Increasing awareness of the environmental pressures on soils and the need to monitor soil health has resulted in a range of potential indicators. Microbial functional diversity is one such indicator that seeks to characterize a core soil attribute. The availability of rapid methods for its assessment will aid our understanding of such pressures on basic soil functioning.