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SOURCES AND ESTIMATED LOAD OF BIOAVAILABLE NITROGEN ATTRIBUTABLE TO CHRONIC NITROGEN EXPOSURE AND CHANGED ECOSYSTEM STRUCTURE AND FUNCTION
Citation:
Jorgensen*, E E., S M. Holub*, P. M. Mayer*, M E. Gonsoulin*, R. G. Silva, A. E. West**, S. J. Tunnell, J. E. Clark, J. L. Parsons, D. M. Engle, AND E. C. Hellgren. SOURCES AND ESTIMATED LOAD OF BIOAVAILABLE NITROGEN ATTRIBUTABLE TO CHRONIC NITROGEN EXPOSURE AND CHANGED ECOSYSTEM STRUCTURE AND FUNCTION. Presented at First Interagency Conf. on Research in the Watersheds, Benson, AZ, October 27 - 30, 2003.
Impact/Purpose:
To inform the public.
Description:
Bioavailable nitrogen is a limiting nutrient throughout the Eastern United States. Research demonstrates that exposure to large doses of nitrogen leads to deleterious environmental impacts. However, effects of chronic exposure to lower doses of nitrogen are not well known. Since 1998, we conducted an integrated multi-disciplinary study investigating ecosystem feedback associated with chronic exposure to low doses of nitrogen. In this nitrogen-limited ecosystem, the ability of the soil system to adapt to new nitrogen inputs was degraded by the first winter season when soil nitrate concentration increased 590%. Increased concentrations of nitrate were especially evident on fertilized plots (P<0.0001) but also were measured on plots that were only fenced to manipulate herbivore abundance (P=0.0443). Changes to the plant, macro invertebrate, herbivore, and microbial communities each reduced ecosystem nitrogen use efficiency thereby producing a self-reinforcing positive feedback loop leading to conditions favoring greater concentrations of soil nitrate. Because all of these effects occur together, it is unclear whether the results seen in soil nitrogen chemistry is from one or some combination of them working together. What is clear however is that changes to trophic feedbacks associated with chronic exposure to low levels of bioavailable nitrogen resulted in an annual mean nitrogen excess of 105.6%. Although it was not our goal to measure mass balance for all nitrogen loss pathways, we estimate that ecosystems similarly exposed to excess or new sources of nitrogen may be at risk and are capable of leaching essentially all new nitrogen additions during seasons of dormancy. These experiments demonstrate that even the relatively small amount of nitrogen deposited in precipitation has the capacity to change multiple aspects of ecosystem structure and function.