SELECTIVE INFLUENCE OF PLANT-SPECIES ON MICROBIAL DIVERSITY IN THE RHIZOSPHERE

Wheat, ryegrass, bentgrass and clover were grown for 4 wk in two diffe rent soil types. In addition, samples of these soils were amended with sucrose to mimic carbon inputs into the soil. At the end of the exper iment rhizosphere microbial communities were extracted. The Biolog(R) system was used to construct sole carbon source utilisation profiles o f these communities. Canonical variate analysis (CVA) was used to anal yse the colour production (OD590) data and to discriminate treatment e ffects. CVA did not differentiate between microbial communities from t he two different soil types. It did, however, produce significant clus tering of microbial communities from the different plant species. Ther e was clear discrimination between the carbon sources utilised by micr obial communities from the different plant rhizospheres. Carbohydrates , carboxylic acids and amino acids were the substrates mainly responsi ble for this discrimination suggesting plants may differ in the exudat ion of these compounds. Isolation and enumeration of culturable organi sms from these soils confirmed the stimulatory effect of the rhizosphe re on microbial growth and in particular pseudomonad proliferation. Th e number of pseudomonads, but not total bacterial numbers, present in the samples and inoculated into the Biolog plate was significantly cor related (r(2) = 0.63, P < 0.01) to colour formation in the wells, indi cating that the Biolog system may reflect Pseudomonas carbon utilisati on preferences. In order to further assess the significance of the tec hnique the results need to be considered in conjunction with knowledge regarding actual carbon sources present in the rhizosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.