Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range |
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Authors: | Eran W Hood Mark W Williams Nel Caine |
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Institution: | (1) Department of Geography and Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309-0450, USA, US |
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Abstract: | Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment
in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along
an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land
cover and basin area affect yields and concentrations of ammonium-N (NH4-N), nitrate-N (NO3-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO3-N decreased downstream from 4.3 kg ha−1 in the alpine to 0.75 kg ha−1 at treeline, while yields of DON were much less variable (0.40–0.34 kg ha−1). Yields of NH4-N and PON were low and showed little variation with basin area. NO3-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation
site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites,
volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R
2 = 0.81, P < 0.001) Concentrations of NO3-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment
than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested
systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties
and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing
vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations
and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation
ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry.
Received 26 July 2001; accepted 6 May 2002. |
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Keywords: | : nitrogen DON DOC surface water chemistry snow biogeochemistry Colorado Front Range |
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