Modeling geogenic and atmospheric nitrogen through the East River Watershed, Colorado Rocky Mountains

Abstract

There is a growing understanding of the role that bedrock weathering can play as a source of nitrogen (N) to soils, groundwater and river systems. The significance is particularly apparent in mountainous environments where weathering fluxes can be large. However, our understanding of the relative contributions of rock-derived, or geogenic, N to the total N supply of mountainous watersheds remains poorly understood. In this study, we develop the High-Altitude Nitrogen Suite of Models (HAN-SoMo), a watershed-scale ensemble of process-based models to quantify the relative sources, transformations, and sinks of geogenic and atmospheric N through a mountain watershed. Our study is based in the East River Watershed (ERW) in the Upper Colorado River Basin. The East River is a near-pristine headwater watershed underlain primarily by an N-rich Mancos Shale bedrock, enabling the timing and magnitude of geogenic and atmospheric contributions to watershed scale dissolved N-exports to be quantified. Several calibration scenarios were developed to explore equifinality using >1600 N concentration measurements from streams, groundwater, and vadose zone samples collected over the course of four years across the watershed. When accounting for recycling of N through plant litter turnover, rock weathering accounts for approximately 12% of the annual dissolved N sources to the watershed in the most probable calibration scenario (0–31% in other scenarios), and 21% (0–44% in other scenarios) when considering only “new” N sources (i.e. geogenic and atmospheric). On an annual scale, instream dissolved N elimination, plant turnover (including cattle grazing) and atmospheric deposition are the most important controls on N cycling.

Spatiotemporal Dynamics of Nitrous Oxide Emission Hotspots in Heterogeneous Riparian Sediments

“…Though the glacial aquifer in this study is heterogeneous with extremely high-K due to the presence of gravel-dominated facies types such as OFG, flow and reactive nitrogen transport are observed to follow similar trends in other systems with lower contrasts in K and less hydraulic connectivity (e.g., . Incorporating sediment heterogeneity and hydrologic perturbations into numerical models of many sites (e.g., Carnevali et al, 2020;Dwivedi et al, 2018;Maavara et al, 2021) would help characterize spatial variability, and significantly improve predictive understandings of biogeochemical processes and their response to perturbations. Buried-valley aquifers, which formed wherever proglacial valleys drained large volumes of sediment and water away from glacial margins, are widespread across parts North America (e.g., Wallace & Soltanian, 2021a) and north-western Europe (e.g., Huuse & Lykke-Andersen, 2000;Jørgensen & Sandersen, 2006) and provide drinking water to nearly 43 million people in the United States alone.…”

“…Though the glacial aquifer in this study is heterogeneous with extremely high‐ K due to the presence of gravel‐dominated facies types such as OFG, flow and reactive nitrogen transport are observed to follow similar trends in other systems with lower contrasts in K and less hydraulic connectivity (e.g., Wallace, Sawyer, Soltanian, & Barnes, 2020). Incorporating sediment heterogeneity and hydrologic perturbations into numerical models of many sites (e.g., Carnevali et al., 2020; Dwivedi et al., 2018; Maavara et al., 2021) would help characterize spatial variability, and significantly improve predictive understandings of biogeochemical processes and their response to perturbations.…”

Understanding the hydrological response of a headwater-dominated catchment by analysis of distributed surface–subsurface interactions

“…The lower parts of LT feature a meandering riparian corridor. The stream is fed by high-energy mountain streams from subcatchments on the LT’s north, namely Copper Creek and Quigley Creek 32 —see Fig. 1 (top).…”

A tale of two catchments: Causality analysis and isotope systematics reveal mountainous watershed traits that regulate the retention and release of nitrogen

“…The balance between the retention and release of nitrogen in headwater catchments is strongly coupled to the hydrological cycle (Maavara et al, 2021;Wan et al, 2021;Schimel et al, 1997;Zhu et al, 2018). The transit times of different solutes through the terrestrial biosphere are dictated by the contact time between water and reactive surfaces including microorganisms (Lansdown et al, 2015;Li et al, 2021;Pinay et al, 2015).…”