Mycorrhizal weathering:: A true case of mineral plant nutrition?

Weatherable minerals in all podzol surface soils and shallow granitic rock under European coniferous forests studied hitherto are criss-crossed by numerous open, tubular pores, 3-10 mu m in width. We hypothesize that these pores were formed by complex-forming, low-molecular weight organic acids exuded by or formed in association with mycorrhizal fungi. It is well known that ectomycorrhizal mycelium represents a greatly extended, and better distributed, surface area for the absorption of nutrients. However, there have been few investigations of how the whereabouts of individual hypha affect nutrient uptake. The results presented here provide direct evidence that the mycelium is able to penetrate, and most probably create, microsites which are inaccessible to plant roots and isolated from bulk soil solution phenomena. Dissolved products could be translocated to the host plant roots, bypassing the soil solution with often toxic concentration of Al3+ from acid rain, and bypassing competition for nutrient uptake by other organisms. Furthermore, there is strong evidence that rock-eating mycorrhizal fungi play a role in the formation of podzol E horizons. The partly speculative interpretations presented here challenge conventional ideas about (1) the importance of nutrient uptake from the bulk soil solution (2) criteria for critical loads of acid atmospheric deposition for forests, and (3) the process of podzolization.