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Fluxes and transformations of nitrogen in a high-elevation catchment, Sierra Nevada

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The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-elevation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasonal snowpack; NO3- and NH4+ were released from storage in the form of an ionic pulse, where the first fraction of meltwater draining from the snowpack had concentrations of NO3- and NH4+ as high as 28 μeq L-1 compared to bulk concentrations of <5 μeq L-1 in the snowpack. The soil reservoir of organic N (81 keq ha-1) was about ten times the N storage in litter and biomass (12 keq ha-1). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification processes produced 1.1 keq ha-1 yr-1 of inorganic N, about 3 1/2 times the loading of N from wet and dry deposition. Less than 1% of the NH4+ in wet and dry deposition was exported from the basin as NH4+. Biological assimilation was primarily responsible for retention of NH4+ in the basin, releasing one mode of H+ for every mole of NH4+ retained and neutralizing about 25% of the annual acid neutralizing capacity produced by mineral weathering in the basin. Nitrate concentrations in stream waters reached an annual peak during the first part of snowmelt runoff, with maximum concentrations in stream water of 20 μeq L-1, more than 4 times the volume-weighted mean annual concentrations of NO3- in wet deposition. This annual peak in stream water NO3- was consistent with the release of NO3- from the snowpack in the form of an ionic pulse; however soil processes occurring underneath the winter snowpack were another potential source of this NO3-. Concentrations of stream water NO3- during the summer growing season were always near or below detection limits (0.5 μeq L-1). © 1995 Kluwer Academic Publishers.

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