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Using element ratios, instead of isotope ratios, increases the risk of fractionation. Already in 1926 Fay warned for the use of Ca/Sr ratio to trace sources of Ca \cite{fay_strontium_1926}.   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}.  A closer look at the data presented in \cite{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 weathering 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