Use of rhizosphere carbon sources in sole carbon source tests to discriminate soil microbial communities

Sole carbon source tests (Biolog(TM)), designed to identify microbial isolates can be used to metabolically fingerprint soil microbial communities, although the carbon source profiles were not selected for this purpose, This paper reports on the use of alternative carbon sources not available in the Biolog(TM) GN plates to characterize soil microbial communities. The carbon sources used are compounds commonly found in plant root exudates and are, therefore, ecologically more relevant and representative of the types of substrates available to microorganisms in rhizosphere soil. The additional carbon sources tested included various phenolic acids, amino acids, carboxylic acids, long chain aliphatics and carbohydrates. In total, 125 different carbon sources were used to discriminate between soil samples from 9 different sites each with three types of grassland vegetation. The growth curves for different groups of carbon sources were all sigmoidal, but the maximum rate of utilization was faster for carbohydrates, amino acids and carboxyIic acids than for amides, phenolic and long chain aliphatic acids. Significant discrimination of soil microbial communities between sites, but not grassland types, was shown and was more distinct using the exudate carbon sources than those in the Biolog GN plate. The use of fewer carbon sources which are more ecologically meaningful constitutes a more efficient and economical technique. (C) 1997 Elsevier Science B.V.