Regional analysis of inorganic nitrogen yield and retention in high-elevation ecosystems of the Sierra Nevada and Rocky Mountains

Yields and retention of dissolved inorganic nitrogen (DIN: NO3- + NH4+) and nitrate concentrations in surface runoff are summarized for 28 high elevation watersheds in the Sierra Nevada of California and Rocky Mountains of Wyoming and Colorado. Catchments ranged in elevation from 2475 to 3603 m and from 15 to 1908 ha in area. Soil cover varied from 5% to nearly 97% of total catchment area. Runoff from these snow-dominated catchments ranged from 315 to 1265 mm per year. In the Sierra Nevada, annual volume-weighted mean (AVWM) nitrate concentrations ranged from 0.5 to 13 muM (overall average 5.4 muM), and peak concentrations measured during snowmelt ranged from 1.0 to 38 muM. Nitrate levels in the Rocky Mountain watersheds were about twice those in the Sierra Nevada; average AVWM NO3- was 9.4 muM and snowmelt peaks ranged from 15 to 50 muM. Mean DIN loading to Rocky Mountain watersheds, 3.6 kg ha(-1) yr(-1), was double the average measured for Sierra Nevada watersheds, 1.8 kg ha(-1) yr(-1). DIN yield in the Sierra Nevada, 0.69 kg ha(-1) yr(-1), was about 60% that measured in the Rocky Mountains, 1.1 kg ha(-1) yr(-1). Net inorganic N retention in Sierra Nevada catchments was 1.2 kg ha(-1) yr(-1) and represented about 55% of annual DIN loading. DIN retention in the Rocky Mountain catchments was greater in absolute terms, 2.5 kg ha(-1) yr(-1), and as a percentage of DIN loading, 72%. A correlation analysis using DIN yield, DIN retention and surface water nitrate concentrations as dependent variables and eight environmental features (catchment elevation, slope, aspect, roughness, area, runoff, soil cover and DIN loading) as independent variables was conducted. For the Sierra Nevada, elevation and soil cover had significant (p < 0.1) Pearson product moment correlations with catchment DIN yield, AVWM and peak snowmelt nitrate concentrations and DIN retention rates. Log-linear regression models using soil cover as the independent variable explained 82% of the variation in catchment DIN retention, 92% of the variability in AVWM nitrate and 85% of snowmelt peak NO3-. In the Rocky Mountains, soil cover was significantly (p < 0.05) correlated with DIN yield, AVWM NO3- and DIN retention expressed as a percentage of DIN loading (%DIN retention). Catchment mean slope and terrain roughness were positively correlated with steam nitrate concentrations and negatively related to %DIN retention. About 91% of the variation in DIN yield and 79% of the variability in AVWM NO3- were explained by log-linear models based on soil cover. A log- linear regression based on soil cover explained 90% of the variation of % DIN retention in the Rocky Mountains.