Influence of mature Douglas fir roots on the solid soil phaseof the rhizosphere and its solution chemistry

Plants can induce significant changes in the rhizosphere through the uptake of water and ions, the exudation of organic compounds and the activities of micro-organisms. The aim of the present study was to assess the influence of tree roots on the chemistry (pH, exchangeable cations, total organic carbon) of both the solid phase of the soil and the soil solutions, extracted by centrifugation, under a mature Douglas fir stand over two distinct seasons (March and September, 1999). The chemical characteristics of either the solid soil phase or the soil solutions of the rhizosphere were found to be different from those of the bulk soil. The cation exchange capacity, base saturation and organic C were all greater in the rhizosphere than in the bulk soil, as a possible result of rhizodeposition, incorporation of decaying root material and micro-organism activity. The concentration of all elements increased in the rhizosphere solutions as compared to the bulk soil solutions, except for P. The pH was lower in the rhizosphere than in the bulk soil for both the solid soil phase and the soil solutions. Despite the greater overall Al concentration of the rhizosphere solutions, as compared to the bulk soil solutions, we suggest that in both, Al toxicity was efficiently restricted by both high Ca + Mg contents and Al complexation with various ligands.