Role of organic nitrogen in the nitrogen cycle of a high-elevation catchment, Colorado Front Range

Here we report on 3 years (1996-1998) of measurements of organic and inorganic nitrogen (N) fluxes to and from Green Lakes Valley, a high-elevation ecosystem in the Colorado Front Range of the Rocky Mountains. Nitrate-N (NO3-N) was the dominant form of N in both precipitation and stream water. Annual precipitation contained 52% NO3-N, 32% ammonium-N (NH4-N), 9% dissolved organic N (DON), and 7% particulate organic N (PON). Annual export of N in streamflow was composed of 70% NO3-N, 4% NH4-N, 14% DON, and 12% PON. Thus the percentage of organic N increased from 16% of total N in precipitation to 26% of total N in streamflow. Subtracting inputs from outputs, Green Lakes Valley always shows net retention of inorganic N. The only form of N that showed net export was DON. DON export was low (0.18 to -0.13 kg ha(-1) yr(-1)), with net export recorded in 2 years and basin retention recorded in 1 year. There was a seasonal pattern in the concentrations of inorganic N (NO3-N + NH4-N) and organic N (DON + PON). Concentrations of inorganic N were similar to 15-25 mu mol L-1 during base flow, increased to similar to 30 mu mol L-1 on the rising limb of the hydrograph during snowmelt runoff, then decreased to -5 mu mol L-1 on the recession limb of late summer, with a return to base flow values in the autumn. In contrast, organic N was 7-15 mu mol L-1 during base flow and decreased to near or below detection limits on the rising limb of the hydrograph, with a gradual but consistent increase on the recession limb and on into the autumn. The amount of N in dissolved organic matter changed over time, with the dissolved organic carbon (DOC):DON ratio decreasing from similar to 45 on the rising limb of the hydrograph to < 20 in the autumn. Spatially, there was a striking difference in the ratios of NO3-N and DON between talus and tundra areas. Nitrate concentrations in surface water draining talus areas were always greater than DON. In contrast, DON concentrations in surface water draining tundra areas were always greater than NO3-N. Concentrations of DON were not significantly correlated with DOC (R-2 = 0.04, p > 0.05), indicating that controls on DON export may be different than controls on DOC export. Our results suggest that the ratio of the annual mass flux of inorganic N to organic N in stream waters may provide a novel index to evaluate the N status of terrestrial ecosystems from various biomes.