Selective influence of plant species on microbial diversity in the rhizosphere

Wheat, ryegrass, bentgrass and clover were grown for 4 wk in two different soil types. In addition, samples of these soils were amended with sucrose to mimic carbon inputs into the soil. At the end of the experiment rhizosphere microbial communities were extracted. The Biolog® system was used to construct sole carbon source utilisation profiles of these communities. Canonical variate analysis (CVA) was used to analyse the colour production (OD590) data and to discriminate treatment effects. CVA did not differentiate between microbial communities from the two different soil types. It did, however, produce significant clustering of microbial communities from the different plant species. There was clear discrimination between the carbon sources utilised by microbial communities from the different plant rhizospheres. Carbohydrates, carboxylic acids and amino acids were the substrates mainly responsible for this discrimination suggesting plants may differ in the exudation of these compounds. Isolation and enumeration of culturable organisms from these soils confirmed the stimulatory effect of the rhizosphere on microbial growth and in particular pseudomonad proliferation. The number of pseudomonads, but not total bacterial numbers, present in the samples and inoculated into the Biolog plate was significantly correlated (r2 =0.63, P<0.01) to colour formation in the wells, indicating that the Biolog system may reflect Pseudomonas carbon utilisation preferences. In order to further assess the significance of the technique the results need to be considered in conjunction with knowledge regarding actual carbon sources present in the rhizosphere.