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Patterns of nitrogen accumulation and turnover in riparian soils along the Green and Yampa Rivers.
Adair, Elizabeth1, Binkley, Dan 1, 1
ABSTRACT- Soil nitrogen pools and fluxes were examined from successional series of riparian forests along the regulated Green River and the free-flowing Yampa River. Total nitrogen increased through time in surface soils and throughout the soil profile along both rivers. Nitrogen accumulated at a rate of about 7.8 g N m-2 yr-1 along both rivers for 10 to 80 years. Accumulation of N then slowed to 2.9 g N m-2 yr -1 from years 80 to 170. Sediment deposition was probably the main source of N in these ecosystems. Available N (as measured by ion exchange resin bags) increased with age along both rivers. This index of available N more than doubled over 150 years. Nitrification rates increased through time to steady or slightly declining rates. While the general trend in nitrogen mineralization and nitrification rates was similar, the time-related trend was not significant. Annual N mineralization and nitrification rates were twice as high along the regulated Green River. Turnover rates were especially high in the cottonwood establishment (0-20 year old) sites along the Green River, compared to similar sites along unregulated Yampa River. This could be due to differences in flow regime, such as a lack of repeated or high-energy scouring floods, and the subsequent development of stable of wetland plant communities on these lower terrace sites. High mineralization rates were reflected by high concentrations of N in cottonwood foliage, indicating high levels of plant N uptake in these sites. Many factors contribute to the alteration of riparian ecosystems along regulated rivers. Understanding how river regulation changes levels of available N, and how this interacts to influence native plant persistence and biodiversity, is crucial to the preservation and management of these valuable ecosystems.
KEY WORDS: nitrogen cycling, nitrogen accumulation, riparian succession, river regulation