WEATHERING OF A MICA BY ROOTS AND RHIZOSPHERIC MICROORGANISMS OF PINE

The weathering of minerals within the rhizosphere has been demonstrated many times; however, only a few studies have been concerned directly with root-microorganism interactions. This study was conducted to determine the influence of symbiotic (mycorrhizal) and nonsymbiotic microorganisms on rhizospheric weathering of mica. A lysimetric experiment was performed in a greenhouse to study the weathering of a mica (phlogopite) by roots and rhizospheric microorganisms of pine (Pinus sylvestris L.). Pine roots were inoculated or not with an acid-producing rhizobacteria (Agrobacterium sp.) and an ectomycorrhizal fungus (Laccaria laccata [Scop.: Fr.] Berk. and Broome). In noninoculated pine, rhizospheric phlogopite particles were smaller than control phlogopite particles and partially transformed into vermiculite. Inoculation of pine roots with the bacteria increased cation-exchange capacity and losses of K from the mica flakes. Losses were attributed to increased production of organic acids in the rhizosphere by the bacteria. Fewer losses of K were observed in pine mycorrhizosphere as monitored by x-ray diffraction. Electron microscopic observation with a microprobe, however, demonstrated that K losses were greater for the phlogopite particles closely attached to the mycorrhizae. The mycorrhizal effect was attributed to an increase of exchange surface area, rather than to an increase in acidification. Dual inoculation significantly promoted mycorrhizal infection and phlogopite cation-exchange capacity, compared with inoculation with the bacterium or the fungus alone.