FLUXES AND TRANSFORMATIONS OF NITROGEN IN A HIGH-ELEVATION CATCHMENT,SIERRA-NEVADA

The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-e levation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasona l 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 mu eg L(-1) compared to bulk concentrations of < 5 mu eq L(-1) in the snowpack. T he soil reservoir of organic N (81 keg ha(-1)) was about ten times the N storage in litter and biomass (12 keg ha(-1)). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification pro cesses produced 1.1 keg ha(-1) yr(-1) of inorganic N, about 3 1/2 time s the loading of N from wet and dry deposition, Less than 1% of the NH 4+ in wet and dry deposition was exported from the basin as NH4+. Biol ogical assimilation was primarily responsible for retention of NH4+ in the basin, releasing one mode of H+ for every mole of NH4+ retained a nd neutralizing about 25% of the annual acid neutralizing capacity pro duced by mineral weathering in the basin. Nitrate concentrations in st ream waters reached an annual peak during the first part of snowmelt r unoff, with maximum concentrations in stream water of 20 mu eq L(-1), more than 4 times the volume-weighted mean annual concentrations of NO 3- in wet deposition. This annual peak in stream water NO3- was consis tent with the release of NO3- from the snowpack in the form of an ioni c pulse; however soil processes occurring underneath the winter snowpa ck 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 mu eq L(-1)).