Nitric acid, particulate nitrate and ammonium in the continental free troposphere: Nitrogen deposition to an alpine tundra ecosystem

Atmospheric sampling of nitric acid vapor, particulate nitrate and ammonium has been ongoing at a 3540 m a.s.l. alpine tundra site on Niwot Ridge, Colorado, since January, 1993. These nitrogen (N) species data, in conjunction with meteorological data, N data at a 3020 m a.s.l. subalpine site, and back-trajectory information, show that over 90% of the sampling was obtained under free tropospheric but, apparently, N-enriched conditions. Seasonal concentration and estimated N dry deposition trends are presented. Dry plus wet atmospheric N loading is found to provide 2.5-3 kg N ha(-1) of new, available N to tundra plants during the growing season (mid-May to mid-September), of which 50%, or more, is dry deposited. During the eight-month, nongrowing season about 5 kg N ha(-1) is wet plus dry deposited to the aggrading snowpack or dormant tundra plants. Up to half of this nongrowing season N loading is made available to growing tundra plants during snowmelt. Thus, a total of 3-5 kg N ha(-1) is annually made available to tundra plants by atmospheric deposition of, primarily, anthropogenically derived N. This annual input of new, available N to nutrient N-limited tundra plants may be compared with the dominant pathway for plant N availability at Niwot Ridge-net mineralization of 10-12 kg N ha(-1) yr(-1) recycled through the soil. The ratio of atmospheric N deposition to net N mineralization, presently approaching 0.5, is higher at the Niwot Ridge alpine tundra ecosystem than at most other ecosystems in the continental U.S.