Coupled Nitrogen and Calcium Cycles in Forest Ecosystems: New Insights from Multiple Stable Isotopes

氮是调节温带森林生态系统植物生产力和钙循环的关键必需养分。氮供应增加超过植物和生态系统需要的阈值，可能会导致森林发生根本性的非线性变化，包括过量的硝酸盐淋滤和土壤中潜在的钙枯竭。这些变化使森林面临前所未有的富氮、贫钙条件，可能影响森林的长期健康和可持续性。这项跨学科研究将利用太平洋西北部沿海道格拉斯冷杉林氮状态的广泛梯度，作为检查氮和钙循环之间耦合的一种方式。钙、锶和氮的自然丰度和富集稳定同位素研究将用于检验：1)钙的大气来源与风化来源对土壤氮梯度生长的森林的相对重要性；2)生态系统氮和钙状态如何影响新钙输入的滞留；锶在植物-土壤-菌根系统中作为钙示踪剂的应用。这项工作的结果将有助于太平洋西北地区森林的健全管理，并将完善对温带森林在日益富氮的世界中如何发挥作用的更广泛预测。

Nitrogen is a key essential nutrient that regulates plant productivity and the cycling of calcium in temperate forest ecosystems. Increases in nitrogen availability beyond the threshold of plant and ecosystem needs may drive fundamental, non-linear changes in forests that include excess nitrate leaching and potential calcium depletion from soils. These changes expose forests to unprecedented nitrogen-rich, calcium-poor conditions that can impact long-term forest health and sustainability. This interdisciplinary research will take advantage of a wide gradient in nitrogen status for coastal Douglas-fir forests of the Pacific Northwest as a way to examine couplings between cycles of nitrogen and calcium. Natural abundance and enriched stable isotope studies of calcium, strontium, and nitrogen will be used to examine: 1) the relative importance of atmospheric vs. weathering sources of calcium to forests growing across a gradient in soil nitrogen; 2) how ecosystem nitrogen and calcium status influences the retention of new calcium inputs, and; 3 the utility of strontium as a tracer for calcium in plant-soil-mycorrhizal systems. Results from this work will contribute to the sound management of forests of the Pacific Northwest, and will refine broader-scale predictions of how temperate forests in general will function in an increasingly nitrogen-rich world.