| Summary: | The two most significant elements with atmospheric components that influence ecological processes at Niwot Ridge are carbon (C) and nitrogen (N). The enrichment of the atmosphere by carbon dioxide (CO2) is ubiquitous across the globe. Global and regional patterns of the annual increases in atmospheric CO2 as well as the current and anticipated vegetation responses are the subject of ongoing analyses (e.g., Schimel 1995, 1998). Hence, except to emphasize the unique Niwot Ridge contribution to the CO2 database, the material presented in this chapter focuses primarily on N, which has a tremendous potential to influence the structure and function of ecosystems (Vitousek et al. 1997). Anthropogenic increases in atmospheric deposition can have profound effects on terrestrial and aquatic ecosystems (Ollinger et al. 1993). Of particular concern are increases in the deposition of nitrogen-containing species, including nonprecipitative, dry-deposited gaseous and particulate N, which can be an important component of the N cycle. These species may act as fertilizer N and can be utilized by vegetation and microbes with little or no energy expenditure. Estimation of the magnitude of N dry plus wet deposition to alpine tundra and subalpine forest ecosystems of Niwot Ridge is integral to understanding N cycling within these systems. This chapter focuses on the estimation of N deposition to the alpine tundra and, to a lesser extent, to the adjacent subalpine forest. The first section presents a brief review of the chemistry of the air environment over Niwot Ridge. The next section discusses the processes of N deposition and exchange with the alpine landscape, source regions for the N in the air over Niwot Ridge, and a procedure for determining atmospheric deposition from ambient air concentrations. Evidence is presented that anthropogenic sources contribute the majority of N in regional air masses and thus to total N deposition. The role that ammonia gas may play in reducing or enhancing N deposition is also described. The ...
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