The Role of Organic Nitrogen in the Nitrogen Cycle of a High-elevation Catchment, Colorado Front Range, Usa

Here we report on three 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 streamwater. 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 two years and basin retention one 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 about 15 to 25 μmol L−1 during baseflow, increased to about 30 μmol L−1 on the rising limb of the hydrograph during snowmelt runoff, then decreased to about 5 μmol L−1 on the recession limb of late summer, with a return to baseflow values in the autumn. In contrast, organic N was 7 to 15 μmol L−1 during baseflow, 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 DOC:DON ratio decreasing from about 45 on the rising limb of the hydrograph to less than 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 constrast, DON concentrations in surface water draining tundra areas were always greater than NO3−N. Concentrations of DON were not significantly correlated with DOC (R = 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 streamwaters may provide a novel index to evaluate the N status of terrestrial ecosystems from various biomes.