A tale of two catchments: Comparing and contrasting nitrogen export patterns from Coal Creek and East River using isotopic signatures

Nitrogen is a limiting element within mountainous ecosystems. Coarse soils, sparse vegetation, and strong hydrological events, such as snowmelt and monsoonal precipitation, can flush nitrogen prior to assimilation and retention in plant and microbial biomass. However, the role discrete landscape properties play in determining the retention and release of nitrogen at the watershed scale is poorly understood. Our study focuses on two catchments within the East River watershed in the Upper Colorado River Basin: Coal Creek and the main stem of the East River. The contrasts between these two catchments allow us to explore the role that catchment characteristics have on solute export. At 56 km2, Coal Creek exhibits an east-west orientation, with north- and south-facing aspects, ~70% land cover by conifer tree species, and bedrock dominated by sedimentary and igneous rock types. The 86 km2 main stem East River above the Pumphouse intensive study site is oriented in a northwest-southeast direction, has 26% coverage by conifer species, and is largely underlain by Cretaceous Mancos shale bedrock, which is entirely absent in Coal Creek. Runoff characteristics for both catchments are similar in terms of the timing of peak discharge in early June and the transition to baseflow in late September-early October, where groundwater represents a significant fraction of streamflow.The relatively smaller catchment size of Coal Creek results in ca. 40% less water export as compared to East River. These distinguishing characteristics manifest themselves in terms of the concentration magnitude and isotopically inferred source of nitrogen exported from each catchment. For example, the nitrate exported from Coal Creek is sourced primarily from new, atmospherically deposited nitrate, which makes up ~54 % of riverine nitrate. By contrast, nitrate is retained longer in the East River watershed, where it is cycled multiple times, and when exported, is primarily (~64 %) derived from terrestrial sources. Taken collectively, our multi-year results provide a unique insight into the primary controls on the provenance of nitrogen loading and its riverine export when considered over scales that functionally aggregate to a collective, integrated watershed response.