Net Ecosystem Exchange, Soil Respiration, and the Age of Respired Carbon from High-Elevation Alpine Tundra

Carbon dioxide (CO₂) is increasing in the contemporary atmosphere as a result of an imbalance between anthropogenic and natural CO₂ emissions and biospheric and oceanic CO₂ assimilation. A comprehensive understanding of the carbon cycle is therefore crucial to assess the effects of present-day emissions, policy decisions, and climate forcing scenarios moving into the future. Alpine tundra is an underrepresented ecosystem in global environmental databases due to its inherently remote nature and the difficulties associated with working in steep, mountainous terrain. Recent work, however, suggests that CO₂ emissions from alpine tundra may be far greater than previously estimated. Toward process-based reconciliation of net annual carbon loss from high-elevation alpine tundra, we evaluated net ecosystem exchange (NEE) and soil respiration (R_S) at Niwot Ridge, Colorado using a unique suite of measurement techniques in combination with ¹⁴C isotopic analysis. Mean annual NEE measured using eddy covariance was approximately 200 g C m^-2 between June 2007 and June 2012. The R_S was variable over both space and time, and reinforced the heterogeneity traditionally associated with physical and physiological processes in alpine areas. The R_S increased with soil moisture until the point of saturation, and the seasonality of peak fluxes was correlated with landscape position as a result. Preliminary ¹⁴C analysis of respired soil air yielded dates between 0 and 5 years old. Notwithstanding, we believe this imbalanced carbon cycle suggests metabolism of paleo-carbon by an unexpectedly active microbial community. Confirmation of this hypothesis could demonstrate that directional climate change has already begun to affect alpine regions, and may provide an early indication of more-widespread carbon cycling patterns in the future.