SG: Continental and Century Scale Assessment of Forest Nitrogen Cycling in the United States Using Dendroisotopes

氮是地球上最重要的物质之一。现代农业依赖于它，因为它是肥料的关键成分。然而，人类使用的大部分氮可以逃逸到水和空气中，在那里它可以对人类和生态系统健康造成问题。尽管由于人类活动，生态系统的氮供应正在增加，但许多生态系统似乎具有很高的能力来保留或去除有害途径中的氮。该项目将通过测量氮循环的长期记录来确定哪些地方具有这种能力，以及是什么导致了这些模式。美国大部分森林中的树木每年都会形成年轮，以保存树木的历史。一棵树吸收的一部分氮被保存在它的木材中，提供了那个地方过去营养条件的记录。这项研究将极大地扩展从木材中的氮中测量的少数几个数据点，并且它将延长研究的时间范围比以前的努力更长。此外，该项目将通过向中学生宣传树木年轮研究、开放数据和研究成果，并提高了对如何更好地管理氮的理解，使其不会对人类或生态系统造成危害。该项目将产生第一个大陆规模的耦合树采样，年轮生长和氮同位素记录的采样树木在47个地点在美国。这将产生关于木材的氮同位素组成和陆地氮循环的三个关键数据集。三个研究目标是：（1）对氮供应的长期模式进行首次大陆规模的重建，特别是测试森林是否正在经历氮供应的增加或减少，（2）确定过去世纪氮供应轨迹的关键机制和驱动因素，特别是难以用其他方法研究的缓慢过程，（3）实现木材氮同位素应用的三个方法学进展。

Nitrogen is one of the most important substances on Earth. Modern agriculture depends on it because it is the key component of fertilizer. However, much of the nitrogen used by humans can escape to water and the air, where it can cause problems for human and ecosystem health. Although nitrogen supply to ecosystems is increasing due to human activities, many ecosystems seem to have a high capacity to retain or remove nitrogen from harmful pathways. This project will identify which places have this capacity, and what is causing these patterns, by measuring long-term records of nitrogen cycling. Trees in most forests of the U.S. form annual growth rings that preserve the history experienced by that tree. A portion of nitrogen taken up by an individual tree is preserved in its wood, providing a record of past nutrient conditions in that place. This research will greatly expand the few individual data points that have been measured from nitrogen in wood, and it will extend the timeframe of study much longer than previous efforts. Further, this project will deliver benefits to society and the larger scientific community through outreach about tree ring research to middle school students, open access of the data and research findings, and increased understanding of how to better manage nitrogen so that it does not cause harm to people or ecosystems.This project will generate the first continental-scale sampling of coupled tree-ring growth and nitrogen isotope records by sampling trees at 47 sites across the United States. This will produce three key data sets about the nitrogen isotopic composition of wood and the terrestrial nitrogen cycle. The three research goals are: (1) to generate the first continental-scale reconstructions of long-term patterns of nitrogen availability, particularly testing whether forests are experiencing increasing or decreasing nitrogen availability, (2) to identify the key mechanisms and drivers of trajectories in nitrogen availability over the past century, especially slow processes that are difficult to study with other approaches, and (3) to accomplish three methodological advances for broader use of wood nitrogen isotopes.