Role of mycorrhizal symbiosis in mineral weathering and nutrient mining from soil parent material.

#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 minerals) 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, and will eventually dissolve completely.
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 et al., 1999).

In addition, mineral weathering produces cations that counteract soil acidification, thereby improving the availability of most plant nutrients (Breemen et al., 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 et al., 1996).

Moreover, weathering of Ca- and Mg-silicate minerals play a central role in the global carbon cycle, because large amounts of Ca and Mg, released by the weathering process, will be locked up as carbonates in marine sediments (Sundquist, 1985).
In 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 et al., 1992).

The vast amounts of nutrients locked in soil minerals triggered, nearly 100 years ago, the question of wether or not plants can actively tap into this potential nutrient source (Haley, 1923; Turk, 1919).
Five decades later, studies appear on the role of microorganisms, including mycorrhizal fungi, in mineral weathering (Webley et al., 1963; Duff et al., 1963; Sperber, 1958; Boyle et al., 1967; Boyle et al., 1973).
More recently, a publication with the provocative title `Rock eating fungi' appeared in the journal Nature (Jongmans et al., 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 et al., 2009; Hoffland et al., 2004; Landeweert et al., 2001).

Since 2009, more evidence of mycorrhizal weathering has been published, based on in vitro and microcosm research.
A new perspective is the influence of the emergence 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 studies 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 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.