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

在温带森林生态系统中，氮是调节植物生产力和钙循环的关键基本养分。氮素有效性的增加超过了植物和生态系统需求的门槛，可能会导致森林发生根本性的、非线性的变化，包括土壤中过量的硝酸盐淋失和潜在的钙消耗。这些变化使森林暴露在前所未有的富氮缺钙条件下，这可能会影响森林的长期健康和可持续性。这项跨学科的研究将利用太平洋西北部沿海道格拉斯冷杉森林的氮素状况的广泛梯度，作为检查氮和钙循环之间的耦合的一种方式。钙、锶和氮的自然丰度和丰富的稳定同位素研究将被用来检查：1)大气和风化的钙源对土壤氮梯度生长的森林的相对重要性；2)生态系统氮和钙状况如何影响新钙输入的保持；以及3锶作为植物-土壤-菌根系统中钙的示踪剂的有效性。这项工作的成果将有助于太平洋西北部森林的健全管理，并将完善对温带森林在日益富含氮素的世界中一般将如何发挥作用的更广泛的预测。

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.