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

Decades of acidic deposition and timber harvesting have depleted calcium (Ca) stocks in soils, especially at base-poor soils characterized by low base cation weathering rates. One approach to tracking Ca sources from soil is by using Ca/Sr ratios in vegetation, while [sup.87]Sr/[sup.86]Sr ratios have also 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 for Ca, Sr, and [sup.87]Sr/[sup.86]Sr ratios. Higher Ca/Sr ratios in soil extracts and sugar maple tissues at base-rich sites compared with base-poor 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, [sup.87]Sr/[sup.86]Sr ratios in soil and tree tissues were higher than precipitation but were indistinguishable for off-shield sites. Mixing models using a 1.0 mol [L.sup.-1] 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. Key words: calcium, strontium isotopes, weathering, deposition, sugar maple
Introduction A century of atmospheric acidic deposition and timber harvesting has reportedly depleted soil calcium (Ca) stocks over large parts of eastern North America (Likens et al. 1996; Watmough et [...]