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Modelling is a useful tool to test the scope of mycorrhizal weathering concepts.   In this section we will review the available field data and modeling work on mycorrhizal weathering in this research field.  Without doubt does vegetation have a substantial positive effect on soil mineral weathering \cite{Berner_1992}. \citep{Berner_1992}.  It remains the question though, how much influence the associated mycorrhizal fungi have on this vegetation effect.  Due to the slow kinetics of soil mineral weathering, and the complex soil matrix, a direct estimation of the contribution of mycorrhizal fungi on the weathering process is challenging.  Three different approaches have been adopted to address the impact of mycorrhizal weathering: 1) historical weathering markers, 2) stable isotopes to trace the source of tree nutrients and 3) quantifying incubated minerals in contrasting soils.  In addition, modeling can be a powerful tool to upscale proposed mycorrhizal weathering mechanisms to soil or even global scale weathering processes.  ##Historical weathering markers  Tunnels, as described in \cite{Jongmans_1997} \citet{Jongmans_1997}  are the only quantifiable fungal markers of weathering that remain visible over geological time. Unfortunately, fungal tunneling either reflects only a small portion of the total effect of fungi on the weathering process, or the fungal impact is negligible, as tunneling contributes less than 0.5% tot total mineral weathering \cite{Smits_2005}. \citep{Smits_2005}.  In a recent paper Koele et al. \cite{Koele_2014} \citet{Koele_2014}  showed that mineral tunneling is not exclusively found under ectomycorrhizal vegetation, but also in forest soils, never exposed to ectomycorrhizal vegetation. This suggests that tunnels can be formed by other means. ##Isotope tracers  Stable isotopes of especially Ca and Sr have been used extensively to source the origin of Ca in drainage water .  
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As apatite is generally only a minor mineral in the soil mineral matrix, its contribution to the soil solution Ca pool is minor compared to other minerals.  If the Ca isotope ratio in the plant is more similar to the signature in apatite than in the soil solution, it indicates that the plant takes up Ca directly from the apatite crystal.   As the apatite crystals are below the root scale, it indicates a selective uptake via mycorrhizal hyphae colonizing apatite grains.   In an influential paper Blum *et al.* \cite{Blum_2002} \citet{Blum_2002}  applied this technique, but as in their study area, the different mineral sources did have similar Ca isotope ratios, they used the ratio between Ca and Sr instead. Using element ratios, instead of isotope ratios, increases the risk of fractionation. Already in 1926 Fay \citet{Fay_strontium_1926}  warned for the use of Ca/Sr ratio to trace sources of Ca \cite{fay_strontium_1926}. Ca.  Most of the Ca taken up by trees comes from litter recycling. In a comparable northeastern mixed forest, the annual Ca import from weathering in the rooting zone is less than 0.3% of the annual Ca uptake , which was a 4 times smaller flux than the annual atmospheric deposition \cite{Dijkstra_2002}. \citep{Dijkstra_2002}.  A closer look at the data presented in \cite{Blum_2002} \citet{Blum_2002}  clearly separates ectomycorrhizal trees with a high Ca/Cr ratio (the two coniferous species) and ectomycorrhizal trees with a low Ca/Sr ratio in their leaves (the two deciduous species). Although in principle this difference could be explained by host specific mycorrhizal communities, with the both coniferous species hosting mycorrhizal fungi with stronger capability to weather apatite, a more obvious explanation is that Ca/Sr fractionation is different during throughfall and litter recycling between these coniferous and deciduous trees.   Up to now, isotope techniques have not provided convincing evidence of a major mycorrhizal contribution in mineral weathering.  ##Mineral incubations  Long-term soil incubation of minerals in mesh-bags is an approach to study the weathering process in situ.  The application of this type of experiments is reviewed by Gobran *et al.* \cite{Gobran_2005}. \citet{Gobran_2005}.  Not included in that review is a more recent study by Turpault et al \cite{Marie_Pierre_2009}. \citet{Marie_Pierre_2009}.  A strong advantage of the incubation approach, compared to microcosm experiments is that weathering rates are measured under real soil conditions.   The drawback is that it is impossible to distinguish mycorrhizal weathering actions from other weathering actions. What is possible is to create root-exclusion zones.   The study by Turpault *et al.* \cite{Marie_Pierre_2009} \citet{Marie_Pierre_2009}  show a halving in Labradorite (a Ca-rich feldspar) dissolution rate in root exclusion zones. This effect diminished in plots that were previously Ca-fertilized. Apatite showed more complex dynamics. In the top soil the apatite dissolution rate is higher in the root exclusion zones, while at 20 cm depth it is the other way around. In contrast to Labradorite liming did not have a strong effect on the apatite dissolution dynamics. Interestingly, the same dynamics of apatite dissolution rate appeared in a very different setting.   Smits *et al.* \cite{Smits_2014} \citet{Smits_2014}  studied apatite loss from the soil profile in a vegetation gradient in Norway. Overall, apatite dissolution rate show a clear correlation with pH.   The factor pH explained 74% of all variation in dissolution rate.   The remaining variation showed in the top soil a negative correlation and at 30-40 cm depth a positive relation with fungal biomass.   A possible explanation, given in \cite{Smits_2014} \citet{Smits_2014}  is that in the top soil mineral dissolution is inhibited by the adsorption of fulvic and humic acids (produced by the rhizosphere induced soil organic matter degradation). This illustrates the strong interrelationship of mineral dissolution with other soil processes.  ##Modeling 
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Due to the limitation of computational power, simulation models are restricted in their spatial scale.   How these small scale mechanisms are upscaled can have major consequences on the model behavior at soil level.  The most well-known weathering model, PROFILE \cite{Sverdrup_1993} \citep{Sverdrup_1993}  is based on a series of chemical dissolution reactions simulated at the scale of soil layer 'bulk' soil solution. The parameterization is based on batch experiments with organic and inorganic weathering agents.  Based on this model, and its application in many boreal forest soils, Sverdrup \cite{Sverdrup_2009} \citet{Sverdrup_2009}  concludes that protons are the major weathering agent in these soils, organic chelators like oxalate only play a minor role. The main critique on the PROFILE model from a fungal point of view is that simulating weathering on a soil layer scale overlooks the potential importance of local, fungal-scale, high concentrations of fungal weathering agents\cite{Finlay_2009}. agents\citep{Finlay_2009}.  But, as illustrated in \cite{Smits_2009}, \citet{Smits_2009},  due to the specific dissolution kinetics of the main dissolution reactions, local concentration of weathering agents do not automatically lead to higher dissolution rates. Following from the kinetics the PROFILE modelling approach only underestimate the action of certain weathering agents if they are exuded to and stay within a micrometer range of specific mineral surfaces.  In contrast to the PROFILE model, the weathering module developed by Taylor *et al.* \cite{Taylor_2011} \citet{Taylor_2011}  incorporates exudation of fungal weathering agents on the mineral grain scale. The model explicitly assumes that in systems with ectomycorrhizal trees all tree-soil interactions (uptake and exudation) takes place via the ectomycorrhizal fungi acting close only to nutrient-bearing minerals (so not close to quartz which is the most common mineral in top soils).   Now, the presence of ectomycorrhizal fungi has a major impact on mineral weathering.  Interestingly, like in the PROFILE model, protons are the major weathering agents (Layla Taylor, personal communication). *personal communication*).  Although this model neatly explains the increased silicate weathering during the rise of ectomycorrhizal fungi over the past 120 million years, we have major issues with the reality of the assumptions made.  The main part of protons exuded by plant roots and mycorrhizal fungi is in exchange for nutrients (cations and NH4+). NH₄⁺).  Most of these nutrient come from organic matter breakdown.  Then most proton are exuded where these nutrients are taken up.  That is mostly at CEC sites (clays and organic matter) or close to the degrading organic matter itself, and not specifically close to nutrient-bearing minerals!  Fungal length measurements on individual grains from the top soil in a boreal pine forest does show a preferential colonization of feldspars over quartz, but as quartz was the dominant mineral, most fungal length was still found on the quartz grains.  Also, mineral mesh bag incubation studies do not show a clear effect of incubated mineral type on fungal colonization rates\cite{Rosenstock_2016}. rates \citep{Rosenstock_2016}.  These observations undermine the validity of the assumption that all ectomycorrhizal exchange processes take place only at nutrient-bearing minerals.