Evaluating the use of Ca/Sr and 87Sr/86Sr ratios to track Ca sources in sugar maple in Ontario

Decades of acidic deposition and timber harvesting have depleted calcium (Ca) stocks in soils. One approach to tracking Ca sources from soil is by using Ca/Sr ratios, while 87Sr/86Sr ratios have been used to estimate mineral weathering rates. To evaluate the uses of Ca/Sr ratios and Sr isotopes in identifying Ca sources in sugar maple (Acer saccharum Marsh) trees, three base-poor sites on the Canadian Shield and three limestone sites in southern Ontario were sampled. Higher Ca/Sr ratios in soil extracts and sugar maple tissues at base rich sites reflect different minerology among regions, while the Ca/Sr discrimination factor between roots and foliage indicated that internal cycling exerts a major control on Ca/Sr ratios in sugar maple. At the three off-shield sites, 87Sr/86Sr ratios in soil and tree tissues were higher than precipitation but were indistinguishable for off-shield sites. Mixing models using a 1.0 M HCl soil extract as the weathering endmember indicated that a lower proportion of weathering-Ca compared with other geochemical approaches. One potential explanation is that the extraction method dissolves more recalcitrant minerals to a greater extent than under field conditions, leading to a higher weathering rate endmember value used in the mixing model.