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

Abstract

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 soil-vegetation 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. ► Atmospheric and soil weathering Ca apportioned in hardwoods on polymineralic soils. ► 87 Sr/ 86 Sr of minerals deduced from soil leachates, soil solutions and stream water. ► Ca mostly originates from weathering due to easily weathered minerals and deep roots. ► Ca shifts toward atmosphere on hilltops due to canopy filtering of dry particulates. ► Forest floor is major available Ca pool for trees due to Ca buildup and recycling.