Response of Subsurface Carbon Transformation and Transport to Changing Mountain Hydrology

Authors

Devon Kerins¹, Kayalvizhi Sadayappan¹, Wei Zhi¹, Pamela L. Sullivan², Kenneth Williams³, Wenming Dong³, Holly Barnard⁴, Rosemary W. H. Carroll⁵, Matthias Sprenger³, James W. Kirchner⁶, Li Li¹* (lili@engr.psu.edu)

Institutions

¹The Pennsylvania State University–University Park, PA; ²Oregon State University–Corvallis, OR; ³Lawrence Berkeley National Laboratory, Berkeley, CA; ⁴University of Colorado–Boulder, CO; ⁵Desert Research Institute, Reno, NV; ⁶Swiss Federal Institute of Technology in Zürich, Switzerland

Abstract

Headwater streams drain about 70% of the United States’ land area and receive fluxes of terrestrial carbon matching net ecosystem production. In many parts of the U.S. Rocky Mountains, warming temperatures are responsible for reduced snowmelt inputs and waning stream discharge, leading to shifting flowpaths and subsurface biogeochemical processes that produce mobile carbon. These intertwining processes challenge the quantification and mechanistic understanding of carbon response to warming. Here, the project asks the question: how does subsurface carbon transformation and transport respond to changing hydrology in a warming climate? Researchers focused on the Coal Creek watershed in Gunnison County, CO, where temperatures have been warming 0.44°C/decade (R²=0.66, p<<0.01). Using stream discharge and dissolved organic and inorganic carbon (DOC and DIC) data, the team calibrated a watershed-scale reactive transport model (HBV-BioRT) from October 2015 to September 2021 to understand shallow/deep flowpaths and corresponding carbon transformation and transport. Subsurface reactions were added incrementally, starting with shallow soil respiration followed by deep carbon respiration and sorption processes. This revealed shallow soil processes primarily contribute to stream DOC and DIC during snowmelt, but respiration in longer and deeper flowpaths is essential to reproduce observed DOC flushing and DIC dilution patterns. The fluxes of DOC and DIC from the shallow soil vary significantly across seasons and peak during snowmelt. Annual carbon fluxes depend mostly on the size of snowmelt. DOC and DIC production rates can increase by 50 to 70%, and export rates can increase by 70 to 90% from dry years to wet years. Conversely, carbon transformation and export from deep groundwater flow remains relatively constant across discharge regimes, but the fraction of deeper carbon export becomes much more pronounced in drier years. This work highlights the sensitivity of carbon production and export rates to changing hydrology in mountain climate, especially snowmelt size. These shifts can have important implications for instream processes, carbon dioxide gas evasion, and carbon cycling in warming mountain catchments.