Lichens mineral weathering

Banfield, J. F., Barker, W. W., Welch, S. A. Taunton, A. (1999). Biological impact on mineral dissolution application of the lichen model to understanding mineral weathering in the rhizosphere. Proceedings of the National Academy of Sciences of the United States of America, 96, 3404-11. 

Rocks colonized by lichens weather faster than uncolonized rock surfaces, as a direct result of metal chelation by lichen substances and organic acids (Fig. 15.3). Fomina et al. and Smits (Chapters 10 and 13, this volume) provide comprehensive discussions of the effects of fungi on mineral weathering processes, and therefore only a brief summary of the effects of lichens on this phenomenon will be presented here. Essentially, lichen colonization can influence the rate of silicate weathering through... 

At present it is difficult to compare the effects of different lichen taxa, or different lichen acids, on bulk silicate mineral weathering rates under field conditions. An abundant literature depicts the physical and chemical distribution of minerals, alteration products and organic components during lichen growth, mostly provided through SEM, EDS, XRD and... 

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

In the microenvironments associated with fungal hyphae and roots, however, concentrations of organic acids may be many times higher than in the bulk soil solution (see discussion above). In such microenvironments, significant acceleration of silicate dissolution is to be expected. Such acceleration has been clearly demonstrated in microbial in microbial cultures (e.g., Eckhardt 1979). An apparent effect of lichens in accelerating basalt weathering in Hawaii was described by Jackson and Keller (1970), however, their interpretation has recently been questioned (Cochran and Berner 1992). It is hard to quantify the overall effect of microenvironments on mineral weathering rates. 

Most soils contain approximately 95% of their dry mass as mineral material and 5% as organic material. This chapter is largely concerned with the mineral material, and organic matter is discussed only so far as it influences the fate of the inorganic elements. The mineral material is derived from the break down of rocks. Embryonic soils are little more than a few millimetres of rock debris colonised by lichens while at the other extreme, in the tropics, tens of metres of weathered material may overlie unaltered rock. In all cases, however, the chemical nature of the soil will be influenced by the geochemical nature of the parent rock. In turn, plant contents tend to reflect soil contents, albeit indirectly. 

Adamo, P. Violante, P. (2000). Weathering of rocks and neogenesis of minerals associated with lichen activity. Applied Clay Science, 16, 229-56. 

Bormann et al. (1998) attempted to determine whether rates of weathering of primary minerals were being underestimated due to failure to include weathering products accumulating in the biomass and in soil. They compared two sandbox ecosystems (large monitored lysimeters) at Hubbard Brook, NH, one with red pine and the other relatively nonvegetated (containing sporadic lichens and mosses). 

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