Weathering, secondary mineral genesis, and soil formation caused by lichens and mosses growing on granitic gneiss in a boreal forest environment

- Lichens and mosses growing on gneiss greatly intensified chemical weathering and mediated clay genesis and soil formation.
- Expandable clay and kaolinite were formed by biotic weathering processes but not by abiotic weathering.
- Mosses caused the most intense weathering and produced the most expandable clay, and kaolinite was produced only by mosses.
- Biotic weathering is attributable to chelation, H2CO3 production, and bioaccumulation of solubilised rock-derived elements.
- Authigenic amphiboles and pyroxenes were produced by mosses, and only by mosses, probably owing to epitaxy on iron chelates.

Lichens and mosses growing on granitic gneiss in a boreal forest environment caused intense chemical weathering of the rock, producing exclusively biogenic secondary minerals and soils and accumulating solubilised rock-forming elements (Si and metals), but weathering in the absence of these organisms was relatively ineffective. Apart from primary biotite, the only phyllosilicates in the rock were sericite and chlorite formed by hydrothermal metamorphism or abiotic weathering, or both, but the biogenic soils contained expandable clay minerals and, in several cases, kaolinite and calcite, together with illite and chlorite. Lichens were especially effective in altering feldspars to clay, but mosses generally produced a greater abundance of expandable clay minerals, and kaolinite was produced only by certain species of moss - specifically the ones whose rock-derived element concentrations and Mg/Ca ratios were highest. Rock-derived element concentrations were usually highest in mosses, and element enrichment increased with the charge/radius ratio of the cation of the element, suggesting carrier-mediated transport across cell membranes. Furthermore, lichens and mosses produced different assemblages of expandable clay minerals, and most mosses (but none of the lichens) produced authigenic amphiboles and pyroxenes. Biological weathering is attributable to solubilisation of rock-forming elements by chelating agents and H2CO3, bioaccumulation of the elements, and nucleation of secondary silicates by chelate complexes functioning as templates for epitaxial growth of specific crystal structures. The different weathering and soil-forming processes of lichens and mosses suggest a mutualistic strategy for maximising the release of inorganic nutrients from rocks and retaining them in bioavailable forms in the soil.