Landscape processes controlling nitrogen loss from mountainous watersheds
Abstract
Nitrogen is often a limiting element within mountainous ecosystems. Coarse soils, sparse vegetation, and strong hydrological events, such as snowmelt and monsoonal precipitation, can flush inorganic and organic nitrogen prior to assimilation and retention in plant and microbial biomass. Here we examined how terrestrial weathering and biogeochemical cycling contribute to the aggregate signal of several years of nitrogen (nitrate, dissolved organic nitrogen, and particulate nitrogen) concentration-discharge data within a pristine (East River) and a metal-contaminated (Coal Creek) catchment within the Upper Colorado River Basin. These paired catchments differ in terms of their bedrock properties. Coal Creek lithology is dominated by granitic rock, while many areas of the East River are underlain by a nitrogen-rich Mancos Shale providing stark contrast to address how bedrock composition can influence nitrogen export. Indeed, we observed that nitrate export from the East River watershed is significantly higher than the Coal Creek, likely due to the influence of bedrock weathering. Within the East River, our work focuses on two distinct spatial scales, a hillslope to floodplain transect, and across the whole watershed (85 km 2 ). By coupling long-term monitoring, field experiments, and mechanistic modeling we constrained a number of terrestrial fluxes for nitrogen including atmospheric deposition, nitrogen fixation, and denitrification. We note that, within river nitrate export generally peaks during the maximum discharge event associated with snowmelt, with minimum nitrate export occurring in the summertime. While this pattern likely represents the seasonal biological activity driving nitrogen retention during the growing season, more complex relationships are noted whereby nitrate exports do not scale linearly with discharge maxima. This presentation will explore this disparity deeper. Overall, this work intends to improve understanding of the feedback between hydrological perturbation (in the formation and loss of snowpack) and biogeochemical processes to improve predictions of nitrogen export at the watershed scale.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B11A..03B
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES;
- 1622 Earth system modeling;
- GLOBAL CHANGE