Constraining soil mineral weathering 87Sr/86Sr for calcium apportionment studies of a deciduous forest growing on soils developed from granitoid igneous rocks

Abstract

Abstracts:We used 87 Sr/ 86 Sr as a proxy for Ca to apportion the contribution of atmospheric deposition and soil mineral weathering sources to the Ca pools in trees in a small forested watershed of southern Quebec. The effects of topography and forest stand composition were assessed by dividing the watershed into three study zones representing two elevations and differences in tree species. Apportionment calculations show that all tree species studied contained Ca that is dominantly sourced from soil mineral weathering (averaging 85%), due to the presence in the soils of easily weathered Ca-bearing minerals such as calcite and apatite as well as the relatively deep rooting habits (reaching the mineral soil) of the tree species. Calcium pools in trees are shifted towards the atmospheric component on hilltops due to the filtering of dry aerosols of the tree canopies which are more exposed to atmospheric circulation than trees at the bottom of the slopes, although some variability in soil bulk chemistry and mineralogy in the watershed obscures the full extent of this phenomenon. The buildup and recycling of Ca in the soilvegetation system, with the forest floor acting as the major accessible Ca pool for the tree roots, must be considered when interpreting apportionment calculations. This work supports other Ca cycling models in forests suggesting that topography can influence the balance of Ca captured from atmospheric sources by trees, even in cases where mineral weathering is the dominant source of Ca nutrition in trees.

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“…Considering previous work on soil mineralogy in St. Hippolyte (Bélanger et al. ), the results can be interpreted by a lower abundance of calcite, apatite, and epidote in the soils of the mixed hardwood–conifer and conifer‐dominated stands because of the acidity that they have produced from their litters and other sources (Finzi et al. , Augusto et al.…”

“…Considering the relatively homogeneous parent material in St. Hippolyte in terms of bulk chemical composition and mineralogy at the time it was deposited by the continental ice sheets (Bélanger et al. , ) and similar soil textures (Table ), the results from the sequential leaching of the B horizon samples suggest that the most easily weathered minerals have been leached from the soil at a faster rate under stands with the presence of conifers, that is, mixed hardwood–conifer and conifer‐dominated stands, compared to hardwood‐dominated stands. This is indicated by lower levels of structural Ca and P in minerals as simulated by HCl leaches of mixed hardwood–conifer and conifer‐dominated stands mineral soils, which is related to greater forest floor acidity (Fig.…”

“…C) suggests that Ca‐rich minerals such as epidote and perhaps calcite (aluminum–iron sorosilicate and calcium carbonate minerals, respectively) are contributing more Ca and less P compared to apatite (pure apatite [CaPO 4 ] has a stoichiometry fixed at a molar Ca/P ratio of 1.66) into the leachates (Bélanger et al. ). The Ca/P ratios of the HCl leaches are also generally similar between forest types, which suggest that the minerals have been congruently leached, despite variations in conifer abundance (Fig.…”

“…The soils were developed from rocky glacial till derived in part from the underlying anorthosite pluton of the Morin series (Doig ), but due to the small size of the pluton, they have a mineral composition that also reflects the mixture of more felsic rocks (e.g., charnockite, mangerite, syenite) in the surroundings of the anorthosite which were mixed by continental ice sheets during the Pleistocene (Bélanger et al. ).…”

“…This approach can also be used to selectively dissolve minerals and, in turn, determine assemblages of minerals in the soil when a more complete chemical analysis of the leaches is performed, for example, total P (Bélanger et al. ). Specifically, organic matter was first removed from the soil by loss on ignition.…”