Landscape-level nitrogen import and export in an ecosystem with complex terrain, Colorado Front Range

Knowledge of import, export, and transport of nitrogen (N) in headwater catchments is essential for understanding ecosystem function and water quality in mountain ecosystems, especially as these ecosystems experience increased anthropogenic N deposition. In this study, we link spatially explicit soil and stream data at the landscape scale to investigate import, export and transport of N in a 0.89 km(2) site at the alpine-subalpine ecotone in the Front Range of the Rocky Mountains, Colorado, U.S.A. For two of the major N inputs to our site, N deposition in the snowpack and N fixation, a complementary relationship was found across the study site, with greater abundance of N-fixing plants in areas with less snow and substantial snow inputs in areas with low N fixer abundance. During the initial phases of snowmelt, mixing model end members for oxygen isotopes in nitrate (NO3 (-)) indicated that a substantial quantity of NO3 (-) is transported downhill into the forested subalpine without being assimilated by soil microbes. After this initial pulse, much less NO3 (-) entered the stream and most but not all of it was microbial in origin. Rising delta N-15 in stream NO3 (-) indicated greater influence of fractionating processes such as denitrification later in the season. NO3 (-) from both atmospheric and microbial sources was not exported from our site because it was consumed within the first several hundred meters of the stream; ultimately, N exports were in the form of dissolved organic nitrogen (DON) and particulate N (PN). The results of this study suggest that the highest elevation dry alpine meadows rely more heavily on N fixation as an N source and experience less of the effects of anthropogenic N deposition than mid and lower elevation areas that have more snow. Our data also suggest that mid-elevation krummholz, moist meadows, and talus slopes are exporting N as NO3 (-) shortly after the onset of snowmelt, but that this NO3 (-) is rapidly consumed as the stream flows through the subalpine forest. This consumption by assimilation and/or denitrification currently provides a buffer against increased inorganic N availability downstream.