SGER: New Insights into Calcium Cycling in Forests through Coupled Ca and Sr Isotopic Measurements

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Calcium (Ca) is an essential forest nutrient that is sensitive to leaching caused by acid rain and nitrogen deposition. Symbiotic fungi may provide plants access to calcium bound in soil minerals, but they too are sensitive to acid rain. Whether leached calcium can be replaced by calcium weathered from soil minerals may accordingly determine how plants adapt to polluted conditions. This issue has not been satisfactorily resolved because researchers have relied on imperfect analogs to study Ca cycling in forest ecosystems, namely strontium/calcium (Sr/Ca) values and Sr isotopic ratios (87Sr/86Sr). Although measurements of isotope ratios often provide powerful insights into internal cycling of elements, we lack basic baseline data on calcium isotope patterns. To examine how Ca and Sr cycling in forest ecosystems are coupled, we will test the partitioning of Ca and Sr elements and isotopes in: 1) cultured pine and birch seedlings with known calcium and strontium sources, and 2) samples carefully selected from field sites across natural and experimental nitrogen deposition gradients. We will specifically address 1) whether Ca isotopes fractionate between parts of trees (e.g., foliage vs. branches), 2) the links between other chemical signatures and Ca isotopic fractionation, 3) how symbiotic fungi affect plant calcium supply and plant calcium isotopes, and 4) how closely Sr/Ca and 87Sr/86Sr document Ca cycling. Seedling results will be compared with similar measurements from long-running nitrogen deposition experiments in hardwood and pine plots at the Harvard Forest Long-Term Ecological Research site. This project will serve the larger scientific community and the health of the nation's forests by evaluating a promising isotopic system for studying a key forest nutrient that is strongly affected by the anthropogenic acid rain prevalent in the northeast, southeast, and California. The project will also serve as the principal project for a female graduate student in biogeochemistry.