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 inorga nic nitrogen (N) fluxes to and from Green Lakes Valley, a high-elevation ec osystem 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. Annua l precipitation contained 52% NO3-N, 32% ammonium-N (NH4-N), 9% dissolved o rganic N (DON), and 7% particulate organic N (PON). Annual export of N in s treamflow was composed of 70% NO3-N, 4% NH4-N, 14% DON, and 12% PON. Thus t he percentage of organic N increased from 16% of total N in precipitation t o 26% of total N in streamflow. Subtracting inputs from outputs, Green Lake s 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 (N O3-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 m u 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 re turn 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):D ON 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 r atios of NO3-N and DON between talus and tundra areas. Nitrate concentratio ns in surface water draining talus areas were always greater than DON. In c ontrast, DON concentrations in surface water draining tundra areas were alw ays greater than NO3-N. Concentrations of DON were not significantly correl ated with DOC (R-2 = 0.04, p > 0.05), indicating that controls on DON expor t may be different than controls on DOC export. Our results suggest that th e ratio of the annual mass flux of inorganic N to organic N in stream water s may provide a novel index to evaluate the N status of terrestrial ecosyst ems from various biomes.