Background/Question/Methods   Global soils contain more C than terrestrial vegetation or the atmosphere, thus changes to patterns of soil C storage and/or efflux have particularly important C cycling ramifications. Alpine tundra soils may provide an exceptional opportunity to study the effects of climate perturbation on soil CO2 efflux since they foster a diverse array of plant and microbial communities, many of which already exist near the edge of their environmental tolerance. This research builds upon previous work that has identified alpine tundra at Niwot Ridge, Colorado as a significant net annual source of CO2 to the atmosphere using eddy covariance. In order to characterize the cause and sustainability of this imbalance, soil C and soil CO2 efflux were measured along an alpine soil moisture and temperature gradient. Principal objectives of this study were to constrain the relative sources (autotrophic vs. heterotrophic) of respired CO2 from this system, determine the overall contribution of soil CO2 efflux to net ecosystem exchange (NEE), and evaluate the relative influence of soil moisture and temperature on soil CO2 efflux across space and time.

Results/Conclusions   Results of this work demonstrate that CO2 is mineralized and respired to the atmosphere year-round at this location despite cold soils and a relatively shallow snowpack. Total ecosystem C loss exceeded growing season C gain by a factor of approximately four between 2007 and 2009. Analysis of soil C demonstrated a potential for persistent, substantial CO2 contribution to the future atmosphere given present-day patterns of NEE. Soil CO2 efflux was correlated to both soil temperature and soil moisture, indicating that currently observed net annual C loss may reflect a perturbation to the magnitude and/or variability of subsurface environmental conditions from steady state. Differential response to soil temperature and moisture between sites, however, highlights that successful modeling of future CO2 efflux from this ecosystem must account for spatial heterogeneity associated with subsurface microsites. Subsequent research in consideration of the age and/or origin of mineralized C substrate would aid understanding of the physical mechanism(s) by which sustained net annual C loss from alpine tundra is made possible.