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  • 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.

    Mechanisms of mineral weathering

    The most visible aspect of weathering is the break up of rocks and minerals into smaller fragments. This so called physical weathering acts on all scales, from the erosion of complete mountain tops to micrometer scale cracks in mineral crystals. Well known mechanisms of physical weathering are thermal stress and mechanical force by freezing water and penetrating tree roots. But also fungal hyphae, colonizing cracks and voids in mineral grains, can produce mechanical force. They can build up high osmotic pressure in their tissues (up to 20 µN µm⁻¹). This is enough pressure to penetrate bullet proof material (Howard et al., 1991), and also to widen existing cracks in mineral grains and rock fragments. The results of physical weathering is an increase in mineral surface area exposed to the soil solution.

    Less visible is the chemical alteration or dissolution of minerals. Although in principle most primary minerals dissolve in soil solution, certain compounds accelerate the process. The most common, and by far quantitatively most important weathering agents are protons. Protons, and also hydroxide under alkaline conditions, attack the ion bindings in the mineral crystal lattice. This process is called hydrolysis (or carbonation when carbonic acid is the main proton donor). Biotic processes have a strong influence on the soil solution pH via the exudation of protons in exchange of positively charges nutrients as NH4+ and K+, the exudation of organic acids and the release of CO2 into the soil solution.

    Organic acids like oxalic acid and citric acid, not only contribute to proton-driven weathering. Their deprotonated anions (in this case oxalate and citrate) interact in a similar way as protons and hydroxide with the mineral crystal lattice. In fact, many of the deprotonated anions of organic acids are stronger weathering agents than protons and hydroxide. They behave as strong complexants with metals including Al3+, a central element in most mineral crystal lattices.

    Another set of organic compounds with metal-complexing properties are siderophores. This type of molecules form strong bindings with especially Fe3+. They play a key role in the release and uptake of Fe into bacteria, fungi and plants (Kraemer et al., 2014; Ahmed et al., 2014). Primary minerals containing substantial amounts of iron, like hornblende and biotite, show enhanced dissolution rates in the presence of microbial or fungal siderophores (Kalinowski et al., 2000; Sokolova et al., 2010).

    To understand the impact of mycorrhizal fungi, we first need to determine what is the limiting step in the dissolution process. After decades of research it is well established that under normal, far from equilibrium conditions, the rate limiting step is the formation of so called activated surface complexes. That is the complexation of weathering agents as protons or organic ligands with metals in the mineral crystal lattice(Furrer et al., 1986; Wieland et al., 1988). The kinetics of this step can be described by the Transition State Theory (TST) (Lasaga, 1984). For a single weathering agent, its effect on dissolution rate can be described with a simple equation: $$R=A⋅k⋅(agent)^n$$ where R is the dissolution rate, A the mineral surface area, k the specific rate coefficient, (agent) the activity of the weathering agent, and n the reaction order. An extremely important notice is that, to our knowledge, for all tested weathering agents on all tested primary minerals, the reaction order is between 0.5 and 0.8. This has major, and counter-intuitive, consequences in understanding the impact of soil solution heterogeneity on soil scale weathering rates, see Smits (2009) and in section 4 of this chapter.

    Fungal weathering in the lab

    Fuelled by carbohydrate supplied by the host, many ectomycorrhizal fungi (EMF) have the capacity to acidify the surrounding substrate and exude organic acids, both when growing in axenic cultures and in symbiosis with plants under laboratory conditions (Rosling, 2009; Hoffland et al., 2004; Schmalenberger et al., 2015). Using flow through systems, Calvaruso et al. (2013) estimated weathering rates to be 10 times higher when ectomycorrhizal pine seedlings were present compared to unplanted systems, and attributed