EFFECT OF INOCULATION WITH BACILLUS POLYMYXA ON SOIL AGGREGATION IN THE WHEAT RHIZOSPHERE: PRELIMINARY EXAMINATION

Abstract The present paper reports the results of a combined physical and microbial approach to studying the aggregation state of a silt loam soil in the wheat rhizosphere. Non-rhizosphere soil aggregate size distribution was attributed to self-fragmentation of clods. In contrast, aggregate size distribution was uni-modal and centered at 0.2-2 mm for soil adhering to wheat roots. On the other hand, inoculation of wheat with a rhizosphere strain of Bacillus polymyxa increased the mass of soil adhering to the roots by 57%. Comparison of aggregate size distributions suggested a more porous structure for the inoculated rhizosphere soil than the uninoculated. An avidin-biotin indirect ELISA procedure, which allows the detection of B. polymyxa in its natural environment, indicated that the population remained at a constant level whatever the treatment or the size of aggregates. Aggregate stability in water, which was low, showed no statistical difference between treatments or between aggregate size fractions. The water-stable aggregates studied here represent the elementary rhizosphere aggregates characterized by the association of clays, silts, fine sands and B. polymyxa cells. No effect on the population level of B. polymyxa or on the stabilization of aggregates was observed after inoculation of soil and seeds. However, a more porous structure developed within the rhizosphere soil. We are continuing to investigate this aspect of the soil structure process which could be related to a spatial extension of the rhizosphere soil. Inoculation by B. polymyxa could play an important role in water retention and nutrient transfer in the rhizosphere through increasing porosity.