Effects of inoculation of EPS-producing Pantoea agglomerans on wheat rhizosphere aggregation

The effect of bacteria secreting an extracellular polysaccharide (EPS) on the physical properties of rhizosphere soil has been investigated as a function of soil water content by using an approach in which wheat seedlings were inoculated with a strain Pantoea agglomerans (NAS206) selected from the rhizosphere of wheat (Triticum durum L.) growing in a Moroccan vertisol. Colonization by strain NAS206 occurred both on the rhizoplane and the root-adhering soil as opposed to the bulk soil. The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant aggregation and stabilization of root-adhering soil, as shown by the combined increases of (i) aggregate mean weight diameter (MWD), (ii) aggregate macro-porosity (pore throat diameter between 10 and 80 mu m), (iii) adhering soil:root mass ratio (RAS/RT), (iv) water-stable > 200 mu m aggregates and (v) 0.1-2 mu m elementary clayey micro-aggregates. Biological exudation and capillary pressure interactions leading to root-adhering soil aggregation are also analysed and discussed.