Metabolic fingerprint of microbial communities from distinct maize rhizosphere compartments

The metabolic abilities of microbial communities extracted from maize rhizosphere and non-rhizosphere compartments were compared by using the Biolog® system. A double sampling protocol (from bulk soil to root surface and along the root) was used for testing the hypothesis that nature and quantity of rhizodeposits could be key factors governing microbial community structure. Rhizoplane microbes are closer to rhizodeposition sites than microbes living in the adhering soil. This proximity generates distinct pools of rhizodeposits between adhering soil and rhizoplane; the carbon pool available for adhering soil microbes is qualitatively and quantitatively different from the original one because of rhizoplane microbial activity. The first sampling was designed to extract microbes from these two compartments. Moreover the different parts of a root are known to release distinct pools of rhizodeposits; the second sampling along the root was designed to extract microbes from the main morphological parts of a root (ramification zone, root hair-elongation zone and apex). Bulk soil without plant (control soil) was also investigated to obtain microbes deprived from rhizodeposits. The comparative metabolic profiling revealed clear shifts between the microbial populations of the rhizoplane and the control soil ones. The main discriminating substrates were carbohydrates, amino acids and amides. Microbial populations from the ramification zone and the control soil were clearly separated from those of the root hair-elongation zone. This discrimination was mainly based on some carboxylic acids, carbohydrates, amino acids and amides. The sampling procedure and the community structure differences revealed in this study confirm a possible structural effect of maize rhizodeposits on telluric microbial populations.