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

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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.