The Influence of Large Herbivores on Nitrogen Cycling Using Natural 15N Abundance

9408771 Frank对稳定同位素自然丰度的分析最近成为了解复杂生态过程的有力工具。 该技术的简单优雅源于对天然存在的稳定同位素的相对容易和廉价的测量，这些同位素提供了关于有机物质的起源和关键化学转化的信息。 在这里，我将应用该方法来研究大型草食动物对通过反硝化和氨挥发的气态氮损失率的影响。 在黄石国家公园，大型食草动物（麋鹿和野牛）增加了15 N的丰度，15 N是一种稀有的氮同位素，相对于14 N，最丰富的同位素，在一个占主导地位的草种。 这一发现，除了了解草食动物如何影响氮循环和氮的同位素组成的变化过程中，表明氮同位素的分析可以提供有关草食动物对反硝化和氨损失率的影响的信息，这两个过程目前是劳动密集型的，昂贵的，有问题的测量。 本研究的目的是进一步描述大型草食动物对放牧草地氮同位素组成的影响，并研究草食动物对这种同位素反应的机制。 具体目标是：（1）测量黄石国家公园土壤和几种主要植物物种的同位素组成，（2）测量食草动物对黄石公园反硝化作用和氨挥发速率的影响，（3）确定食草动物尿液和粪便改良剂对土壤和植物同位素组成的影响。 大型食草动物对氮循环有许多间接的影响，远远超出了它们对植物物质的直接去除。 由于地球陆地表面的50%是牧场，包括草地，稀树草原和开放的林地，支持放牧哺乳动物，食草动物可能对全球氮收支产生深远的影响。 本研究将探讨食草动物对放牧生境气态氮损失的长期影响，以及利用15 N丰度研究陆地生态系统氮循环的适用性。

9408771 Frank Analysis of the natural abundance of stable isotopes has recently emerged as a powerful tool to understand complex ecological processes. the simple elegance of the technique stems from relatively easy and inexpensive measurements of naturally occurring stable isotopes providing information on the origin and key chemical transformations of organic matter. Here I will apply the method to study the effect of large herbivores on rates of gaseous nitrogen loss via denitrification and ammonia volatilization. In Yellowstone National Park, large herbivores (elk and bison) increase the abundance of 15N, a rare nitrogen isotope, relative to 14N, the most abundant isotope, in a dominant grass species. This finding, in addition to an understanding of how herbivores impact nitrogen cycling and during which transformations the isotopic composition of nitrogen is altered, suggests that analysis of nitrogen isotopes may provide information on the effect of herbivores on rates of denitrification and ammonia loss, two processes that are currently labor intensive, expensive, and problematic to measure. The goals of the study are to further describe the influence of large herbivores on the nitrogen isotope composition of grazed grassland, and examine the mechanisms responsible for this isotopic response to herbivores. Specific objectives are: (1) measure isotopic composition of soil and several dominant plant species in Yellowstone National Park, (2) measure the effect of herbivores on rates of denitrification and ammonia volatilization in Yellowstone, and (3) determine the effect of herbivore urine and dung amendments on the isotopic composition of soil and plants. Large herbivores have many indirect effects on nitrogen cycling that go well beyond their direct removal of plant material. Since up to 50% of the Earth's terrestrial surface is rangeland, including grassland, savanna, and open woodland that support grazing mammals, grazers may have a profound effect on the global nitrogen budget. This study will examine the long-term effects of herbivores on gaseous nitrogen loss from grazed habitat, and the applicability of using 15N abundance to study nitrogen cycling in terrestrial ecosystems.