Fungal triggered iron translocation in the oxic environment - Advances in understanding podsolization

Podsolization is influenced by soil forming factors such as precipitation, parent material, variation in e.g., snowpack melt, groundwater table, water flow velocity, and pH. However, in a thorough meta-analysis of 48 publications, containing 259 dated profiles, Zwanzig and co-authors used the formation of an E-horizon and its increase in thickness over time as an indicator of progressive podsolization¹ and showed that progressive podsolization was linked to “Coniferous” and “Ericaceae-Coniferous-Mix” vegetation. Both vegetation types are affiliated with mycorrhizal fungi, which obtains energy from the host plant and in return deliver nutrients to the plant. Specifically, Coniferous and Ericaceae vegetation hosts ectomycorrhizal (ECM) and ericoid mycorrhiza (ERM) fungi, respectively. Linking ECM fungi to the podsolization process was done more than two decades ago by Van Breemen and co-authors². They observed extensive tunnel weathering of primary minerals by ECM fungi in the top horizons which was almost absent in the underlying B-horizon, thus indicating that some ECM fungi accelerate mineral weathering in the top part of the soil and increase Al mobilization. Furthermore, within the last two decades it has been discovered that ECM fungi with diverse evolutionary origins have a large capacity to reductively dissolve iron minerals as a part of their decaying mechanism driven by Fenton chemistry³. Furthermore, genomic analyses suggest that ERM species might have the ability to initiate a Fenton reaction⁴ albeit this has only been experimentally verified for one species⁵. Here it is suggested that the ability to generate tunnel weathering and reductively dissolve iron minerals are not driven by the same mechanism, nor by the same metabolites, but can be done by (at least) two taxonomically distinct yet functionally similar groups of fungi affiliated nutrient pore soils.  Thus, it is suggested that podsolization, or at least Fe translocation in podzol, is driven by mycorrhiza fungi and is an artifact of the fungi decaying mechanism.

Zwanzig, L., Zwanzig, M. & Sauer, D. Outcomes of a quantitative analysis of 48 soil chronosequence studies in humid mid and high latitudes: Importance of vegetation in driving podzolization. CATENA 196, 104821 (2021).

van Breemen, N., Lundström, U. S. & Jongmans, A. G. Do plants drive podzolization via rock-eating mycorrhizal fungi? Geoderma 94, 163–171 (2000).

Tunlid, A., Floudas, D., Op De Beeck, M., Wang, T. & Persson, P. Decomposition of soil organic matter by ectomycorrhizal fungi: Mechanisms and consequences for organic nitrogen uptake and soil carbon stabilization. Front. For. Glob. Chang. 5, (2022).

Martino, E. et al. Comparative genomics and transcriptomics depict ericoid mycorrhizal fungi as versatile saprotrophs and plant mutualists. New Phytol. 217, 1213–1229 (2018).

Burke, R. M. & Cairney, J. W. G. Carbohydrate oxidases in ericoid and ectomycorrhizal fungi: a possible source of Fenton radicals during the degradation of lignocellulose. New Phytol. 139, 637–645 (1998).