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#Introduction
Rocks are the primary source of all plant nutrients, except nitrogen. These nutrients are bound into a variety of crystalline structures (minerals). Minerals are either formed during rock formation from magma (primary mineral) or formed during soil formation (secondary minerals). Secondary minerals are formed when the local soil solution is saturated in respect to that mineral. In contrast to secondary minerals, primary minerals are formed in the earth mantle at high temperature and pressure. At the earth surface these minerals may be thermodynamically unstable. Here, in interaction with water, they either dissolve completely (congruent dissolution) or dissolve partly, leaving a solid residue like clay minerals (incongruent dissolution). This dissolution process is extremely slow for most minerals. It has been estimated that it takes more than 30 million years to dissolve a 1 mm diameter quartz grain under natural soil conditions Lasaga 1984. Nonetheless, soil mineral weathering provides an essential input of plant nutrients into ecosystems, avoiding or delaying nutrient limitations Chadwick 1999.
In addition, mineral weathering produces cations that counteract soil acidification, thereby improving the availability of most plant nutrients Breemen 1983. Also clays are formed as a weathering product of feldspars and micas Oades 1988. Clay particles contribute, with their negative charged surfaces, to the cation exchange capacity (CEC) of the soil, reducing the leaching of positively charged nutrients like K+ and NH4+. Clay content correlates positively with water holding capacity and soil organic matter (SOM) content Sollins 1996.
Moreover, the weathering of Ca- and Mg-silicate minerals play a central role in the global carbon cycle. The Ca and Mg, released by the weathering process, will be locked up as carbonates in marine sediments Sundquist 1985. On the long-term, atmospheric CO2 is regulated by the weathering rates of these minerals, which is influenced by climate and mountain uplift Berner 2003 Raymo 1992.
The vast amounts of nutrients locked in soil minerals has triggered, nearly 100 years ago, the question if plants actively enter this potential nutrient source HALEY 1923 TURK 1919. Five decennia later, studies appear on the role of microorganisms, including mycorrhizal fungi, in mineral weathering WEBLEY 1963DUFF 1963 Sperber 1958 Boyle 1967 Boyle 1973. More recently, a publication with the provocative title Rock eating fungi appeared in Nature Jongmans 1997. This publication presented evidence of, presumably mycorrhizal, fungal hyphae drilling their way (chemically and/or physically) into feldspar grains. This paper initiated renewed interest into the topic. A series of reviews has been published since then, covering the research up to 2009 Finlay 2009 Hoffland 2004 Landeweert 2001.
Since 2009, more evidence of mycorrhizal weathering has been published, based on *in vitro* and microcosm based research. A new aspect is the effect of the arise of different types of mycorrhizal fungi during the evolution of land plants on mineral weathering rates, and thus the global carbon cycle. The gap between laboratory based studies and the real world has been bridged by a number of field based studied and mathematical modeling. So far, evidence of a substantial role of mycorrhizal fungi on soil mineral weathering has been missing, while modeling studies show contrasting results.
In this book chapter we briefly introduce the basics of physical and chemical weathering mechanisms, as insight in these mechanisms is of vital importance in the interpretation of results from laboratory based experiments and modeling studies. Next, we give an overview of the recent literature on this topic, and set their results in perspective with the current knowledge on mineral dissolution kinetics.
