The dynamics of many terrestrial and aquatic ecosystems are controlled by the availability of nitrogen (N), which is often in short supply in the environment. Human activities are increasing N availability in ecosystems both by increasing N inputs through fertilizer addition or atmospheric deposition, and by speeding up internal recycling of N as a result climate warming or landscape modification. While more N can increase biological productivity, this has a significant impact on the structure and function of natural ecosystems that have been shaped by low nutrient conditions for thousands of years. Recently, researchers have sought to use stable isotopes of N --a measurement method that can distinguish between different forms of N-- in plant leaves to understand ecosystem N dynamics. Relatively straightforward measurements of leaf N isotopes at a single point in time record the net effect of multiple complex biological and physical transformations that occur over time as N moves throughout whole ecosystems. This doctoral dissertation research project will make mechanistic measurements of N cycling across a range of Alaskan black spruce forests to provide an underpinning to the interpretation of leaf N isotope made at the same sites. Here, natural gradients of soil fertility in combination with experimental fertilizer additions and analytical modeling will be used to explore and interpret the patterns of leaf N isotopes. These results will help inform the response of natural ecosystems to a changing N cycle. The graduate student, in addition to performing research, will be mentoring and undergraduate research assistant. This study will help better understand the impact of climate change on natural ecosystems.
