Translocation of metals by trees and fungi regulates pH, soil organic matter turnover and nitrogen availability in acidic forest soils

In this paper we provide support for the hypothesis that trees and fungi modify their physical environment in acidic forest soils by actively translocating metal cations from the mineral to the organic horizon. We suggest that pH buffering and soil organic matter (SOM) turnover in organic horizons are regulated by Ca and Mg cycling via the tree canopy and litter fall and by fungal translocation of Al from mineral soil. Fungi in addition translocate Fe from mineral to organic horizons to enhance the degradation of aromatic structures in SOM. Together these processes are in control of N bioavailability, including new inputs via N fixation. Cycling of Ca and Mg via tree canopy typically increase the base saturation and pH towards the surface of organic forest soil horizons. This process is most clearly depicted at sites with laterally moving groundwater. An up-transport of Al from mineral to the organic horizon is most important as a pH buffering process in less productive, acid soils having a pH below approximately 4.5. At this pH, the non-acidic properties of organically complexed Al keep pH at a level sufficiently high for maintained microbial activity. Furthermore, the formation of bridging bonds between major organic functional groups (carboxyls and phenols) and di- and trivalent metal cations of Ca, Al and Fe possesses a strong influence on the tertiary structure of SOM and its persistence to degradation and delivery of N.