Biological mobilization of potassium from clay minerals by ectomycorrhizal fungi and eucalypt seedling roots

Growth, proton and oxalate efflux, K absorption and mineral depletion by one isolate of Cenococcum geophilum Fr., by a putative isolate of Pisolithus microcarpus and by two isolates of Pisolithus sp. were compared in pure liquid culture using either vermiculite or phlogopite as sole K source. An isolate from China (Pisolithus strain XC1) showed greatest biomass and K accumulation from both minerals. There were inverse correlations between reduction in HCl-extractable K and fungal proton efflux (r = -0.912, P < 0.001) and between the ratio (total K lost from the substrate: K accumulation in hyphae) and oxalate efflux (r = -0.890, P < 0.001). Protons effused from the fungi may have replaced interlayer K, and oxalate effused may have led to biological weathering of the minerals, especially under conditions of limited exchangeable K with phlogopite as K source. Pisolithus sp. and Pisolithus XCl gave high depletion rates of CH3COONH4- and HCl-extractable K from the minerals and were therefore selected to study the growth and mobilization and utilization of K from the minerals by ectomycorrhizal eucalypt (Eucalyptus globulus Labill.) seedlings. Mycorrhizal seedlings extracted more K from the two minerals than did non-mycorrhizal controls, and those colonized by Pisolithus XC1 accumulated much more K than those inoculated with Pisolithus sp., a similar trend to fungal K accumulation in vitro. Non-mycorrhizal seedlings extracted very little HF-HClO4-extractable K from the minerals following sequential extraction with H2O, CH3COONH4 and HCl. In contrast, mycorrhizal seedlings significantly depleted K, although there was no difference between the two fungal strains. Patterns of depletion of sequentially extractable K fractions in the minerals by the fungi in pure culture in vitro differed from those exhibited by the fungus-root associations.