Do plants drive podzolization via rock-eating mycorrhizal fungi?

Weathering and supply of nutrients derived from minerals to plants is known to be stimulated by plant symbiotic mycorrhizal fungi. Nutrients are gener ally thought to pass the bulk soil solution before plant uptake. Jongmans e t al. [Jongmans, A.G., van Breemen, N., Lundstrom, U.S., van Hees, P.A.W., Finlay, R.D., Srinivasan, M., Unestam, T., Giesler, R., Melkerud, P.-A., Ol sson, M., 1997. Rock-eating fungi. Nature, 389, 682-683] showed that (ectom ycorrhizal) fungi drill innumerable narrow cylindrical pores (diameter 3-10 mu m) into weatherable minerals in podzol E horizons. The fungi probably f orm micropores by exuding strongly complexing low-molecular weight organic acids at their hyphal tips, causing highly local dissolution of Al silicate s. Micropores occurred in all thin sections of podzols under Pinus sylvestr is and Picea abies available from Sweden (3), Finland (2), Switzerland (2), Denmark (2) and the Netherlands (3), but not in the few available thin sec tions of non-podzolic soils under broadleaves. Many weatherable minerals in the podzol E horizon appeared to be perforated, as opposed to few if any i n the abruptly underlying B horizon, suggesting a link to podzolization. Hi gh concentrations of Al and Si in organic surface horizons under boreal and temperate conifers can be explained by transfer by hypha of weathering pro ducts from the minerals to mycorrhizal roots in the O horizon, followed by release of weathering products that are not taken up by the plants. Rock-ea ting ectomycorrhizal fungi suggest a more direct role for plants in podzoli zation than hitherto realized, providing tight coupling between podzolizati on and mineral weathering. Preliminary observations, however, indicate that mycorrhizal fungi do not play a role in podzolization under Kauri ( Agathi s australis) in New Zealand. (C) 2000 Elsevier Science B.V. All rights rese rved.