EAGER: Establishing Techniques of Dendronitrogen Measurement

在这个由化学系环境化学科学计划资助的项目中，保罗·R·亚利桑那大学的谢泼德正在建立一种技术来测量树木年轮中的氮含量。树枝状氮的数量与生态系统的氮可用性相关，因此可以长期分析养分的可用性。谢泼德教授正在研究克服两个挑战的方法，以了解树木年轮之间氮的流动性。首先，一些含氮物种在包括树木年轮在内的植物组织中是移动的，因此需要确定移动的与非移动的物种的比率。其次，氮很难准确和精确地测量整个木材样品。这些困难在历史上造成了树枝状氮数据的准确性和相关的树枝状氮的时间变化的氮的环境可用性的不确定性。谢泼德教授正在努力建立一个木材预处理策略，将允许测量，反映氮的可用性在环形成的时间，并制定协议，准确测量木材样品中的氮含量。树木年轮中有一种可测量的氮成分，它以蛋白质的形式固定在细胞壁中。 该不动组分可以反映环形成时的环境氮可用性。木材可以进行预处理以有效地去除移动的氮，同时保留非移动的组分。这是通过两个步骤实现的。首先，使用旋转切片机将树芯切向切成薄片，这是一种新颖的方法。第二，然后在超声浴中用有机溶剂的温和组合处理这些切片，以除去移动的氮，同时使管胞细胞保持完整和未受损。这是保持细胞壁中所含的结构性、固定性氮的关键。燃烧元素分析和同位素比质谱法，特别是氮优化，用于定量评估作为时间的函数的树枝状氮的量。这项研究直接影响了我们对森林和生态系统健康和动态的理解，并提高了我们对人类社会对生态系统健康影响的普遍认识。

In this project funded by the Environmental Chemical Sciences Program in the Chemistry Division, Professor Paul R. Sheppard of the University of Arizona is establishing techniques to measure the levels of nitrogen present in tree rings (dendronitrogen). The quantity of dendronitrogen correlates with nitrogen availability to the ecosystem and therefore permits the long term profiling of nutrient availability. Professor Shepard is studying ways to overcome two challenges to understanding the mobility of nitrogen between tree rings. First, some nitrogen-containing species are mobile across plant tissues that include tree rings, so the ratio of mobile to immobile species needs to be determined. Second, nitrogen is difficult to measure accurately and precisely in whole wood samples. These difficulties have historically cast uncertainty on the accuracy of dendronitrogen data and correlating the temporal variation in dendronitrogen to the environmental availability of nitrogen. Professor Shepard is working to establish a wood pretreatment strategy that will permit measurements that reflect availability of nitrogen at the time of ring formation and to develop protocols for accurately measuring nitrogen content in wood samples. There is a measurable component of nitrogen in tree rings that is immobile in the form of proteins within cell walls. That immobile component can reflect environmental nitrogen availability at the time of ring formation. Wood can be pretreated to effectively remove mobile nitrogen while preserving the immobile component. This is achieved in two steps. First, the tree cores are sliced into thin sections in the tangential orientation using a rotary microtome, a novel approach. Second, these slices are then treated with a mild combination of organic solvents in a sonication bath, to remove mobile nitrogen, while allowing the tracheid cells to remain intact and undamaged. This is key to preserving the structural, immobile nitrogen contained in the cell walls. Combustion elemental analysis and isotope-ratio mass spectrometry, specifically optimized for nitrogen, are used to quantitatively assess the amount of dendronitrogen as a function of time. This research directly impacts our understanding of forest and ecosystem health and dynamics, and improves our general awareness about impacts of human society on ecosystem health.