EAGER: Establishing Techniques of Dendronitrogen Measurement

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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.