EAGER: Establishing Techniques of Dendronitrogen Measurement

在这个由化学学部环境化学科学项目资助的项目中，亚利桑那大学的Paul R. Sheppard教授正在建立一种技术来测量树木年轮中存在的氮含量（树氮）。树突氮的数量与生态系统的氮有效性相关，因此可以对养分有效性进行长期分析。谢泼德教授正在研究如何克服两个挑战，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可移动的，因此需要确定可移动物种与不可移动物种的比例。其次，氮在整个木材样品中难以准确和精确地测量。从历史上看，这些困难给树氮数据的准确性以及将树氮的时间变化与环境中氮的可用性联系起来带来了不确定性。Shepard教授正在努力建立一种木材预处理策略，该策略将允许测量反映环形成时氮的可用性，并制定准确测量木材样品中氮含量的协议。树木年轮中有一种可测量的氮成分，它在细胞壁内以蛋白质的形式固定不动。这种不动成分可以反映环形成时的环境氮有效性。木材可以进行预处理，以有效地去除流动氮，同时保留不动成分。这可以通过两个步骤实现。首先，使用旋转切片机（一种新颖的方法）在切向方向上将树核切成薄片。其次，这些切片然后在超声浴中用温和的有机溶剂组合处理，以去除流动氮，同时使管状细胞保持完整和不受损。这是保存细胞壁中结构性的、不可移动的氮的关键。燃烧元素分析和同位素比质谱法，专门针对氮优化，用于定量评估树突氮的数量作为时间的函数。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，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.