author = {R. J. Howard and M. A. Ferrari and D. H. Roach and N. P. Money},
title = {Penetration of hard substrates by a fungus employing enormous turgor pressures.},
journal = {Proceedings of the National Academy of Sciences}
}" data-bib-key="Howard_1991" contenteditable="false">
, 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 class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Kraemer_2014,
doi = {10.1007/s10498-014-9246-7},
url = {http://dx.doi.org/10.1007/s10498-014-9246-7},
year = 2014,
...
author = {Stephan M. Kraemer and Owen W. Duckworth and James M. Harrington and Walter D. C. Schenkeveld},
title = {Metallophores and Trace Metal Biogeochemistry},
journal = {Aquatic Geochemistry}
}" data-bib-key="Kraemer_2014"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Kraemer_2014">Kraemer 2014 class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Ahmed_2014,
doi = {10.1111/1751-7915.12117},
url = {http://dx.doi.org/10.1111/1751-7915.12117},
year = 2014,
...
author = {E. Ahmed and S. J. M. Holmström},
title = {Siderophores in environmental research: roles and applications},
journal = {Microbial Biotechnology}
}" data-bib-key="Ahmed_2014"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Ahmed_2014">Ahmed 2014. Primary minerals containing substantial amounts of iron, like hornblende and biotite, show enhanced dissolution rates in the presence of microbial or fungal siderophores class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Kalinowski_2000,
doi = {10.1016/s0016-7037(99)00371-3},
url = {http://dx.doi.org/10.1016/s0016-7037(99)00371-3},
year = 2000,
...
author = {B.E Kalinowski and L.J Liermann and S.L Brantley and A Barnes and C.G Pantano},
title = {X-ray photoelectron evidence for bacteria-enhanced dissolution of hornblende},
journal = {Geochimica et Cosmochimica Acta}
}" data-bib-key="Kalinowski_2000"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Kalinowski_2000">Kalinowski 2000 class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Sokolova_2010,
doi = {10.1134/s106422931010008x},
url = {http://dx.doi.org/10.1134/s106422931010008x},
year = 2010,
...
author = {T. A. Sokolova and I. I. Tolpeshta and I. V. Topunova},
title = {Biotite weathering in podzolic soil under conditions of a model field experiment},
journal = {Eurasian Soil Sc.}
}" data-bib-key="Sokolova_2010"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Sokolova_2010">Sokolova 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 class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Furrer_1986,
doi = {10.1016/0016-7037(86)90243-7},
url = {http://dx.doi.org/10.1016/0016-7037(86)90243-7},
year = 1986,
...
author = {Gerhard Furrer and Werner Stumm},
title = {The coordination chemistry of weathering: I. Dissolution kinetics of $\updelta$-Al2O3 and {BeO}},
journal = {Geochimica et Cosmochimica Acta}
}" data-bib-key="Furrer_1986"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Furrer_1986">Furrer 1986 class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Wieland_1988,
doi = {10.1016/0016-7037(88)90178-0},
url = {http://dx.doi.org/10.1016/0016-7037(88)90178-0},
year = 1988,
...
author = {Erich Wieland and Bernhard Wehrli and Werner Stumm},
title = {The coordination chemistry of weathering: {III}. A generalization on the dissolution rates of minerals},
journal = {Geochimica et Cosmochimica Acta}
}" data-bib-key="Wieland_1988"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Wieland_1988">Wieland 1988. The kinetics of this step can be described by the Transition State Theory (TST) class="squire-citation ltx_cite" class="ltx_cite" data-bib-text="@article{Lasaga_1984,
doi = {10.1029/jb089ib06p04009},
url = {http://dx.doi.org/10.1029/jb089ib06p04009},
year = 1984,
...
author = {Antonio C. Lasaga},
title = {Chemical kinetics of water-rock interactions},
journal = {J. Geophys. Res.}
}" data-bib-key="Lasaga_1984"style="cursor: pointer" contenteditable="false">class="au-cite-link" href="#Lasaga_1984">Lasaga 1984. For a single weathering agents, agent, its effect on the weathering dissolution rate can be described with:
Application with a simple equation:
\(R=A\cdot k\cdot\left(agent\right)\left(agent\right)^n\)
where R is the dissolution rate, A the mineral surface area, k the specific rate coefficient,(agent) the activity of the
TST weathering agent, and n the reaction order. An extreme 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
mineral dissolution studies including organic complexants, show that understanding the impact of soil solution heterogeneity on soil scale weathering rates, see Smits 2009 and in the section 'From lab to
equilibrium: saprolith, big rock fragments.
