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

钙是森林中的一种必需养分，对酸雨和氮素沉积引起的淋溶十分敏感。共生真菌可能会为植物提供土壤矿物质中结合的钙，但它们也对酸雨敏感。淋溶出的钙能否被土壤矿物质风化的钙所取代，可能因此决定植物如何适应受污染的环境。这一问题一直没有得到很好的解决，因为研究人员一直依赖不完善的类比来研究森林生态系统中的钙循环，即锶/钙(Sr/Ca)值和锶同位素比值(87Sr/86Sr)。尽管对同位素比率的测量往往提供了对元素内部循环的强大洞察力，但我们缺乏关于钙同位素模式的基本基线数据。为了研究森林生态系统中钙和锶的循环是如何耦合的，我们将在以下方面测试钙和锶元素和同位素的分配：1)培养的已知钙和锶来源的松树和桦树幼苗，以及2)在自然和实验氮沉积梯度上精心挑选的田间样品。我们将具体讨论1)树木部分之间的钙同位素是否分离(例如，树叶和树枝)，2)其他化学特征与钙同位素分离之间的联系，3)共生真菌如何影响植物钙供应和植物钙同位素，以及4)锶/钙和87锶/86锶记录钙循环的密切程度。苗木结果将与哈佛森林长期生态研究基地硬木和松树地块的长期氮沉积实验中的类似测量结果进行比较。该项目将通过评估一种有前景的同位素系统来服务于更大的科学界和全国森林的健康，该系统用于研究一种关键的森林养分，这种养分受到东北部、东南部和加利福尼亚州普遍存在的人为酸雨的强烈影响。该项目还将作为生物地球化学专业一名女研究生的主要项目。

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.