LAUSR

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.