Spatio-temporal patterns of soil respiration and the age of respired carbon from high-elevation alpine tundra
PhD: University of Colorado Boulder, 2015.
Fossil fuel combustion is increasing atmospheric carbon dioxide (CO2), which has augmented the natural greenhouse effect, and is likely to increase flooding, permafrost degradation, hurricanes, and loss of sea-ice and glaciers over at least the next 1,000 years. Since feedbacks between the carbon cycle and the climate system are critical for projecting climate change, it is necessary to constrain the biotic response to climate change in order to accurately forecast the consequences of ongoing CO2 emissions. To advance the understanding of carbon cycling in the mountains, this research characterized the spatio-temporal variability of soil respiration across a broadly representative snow-scoured alpine tundra soil moisture and vegetation gradient, within the footprint of ongoing eddy covariance measurements at Niwot Ridge, Colorado, USA. During the growing season, moisture limitation superseded temperature limitation to soil respiration, and increasing soil moisture invoked a bidirectional soil respiration response from areas of fellfield and dry meadow tundra (directly proportional) compared to moist and wet meadow tundra (inversely proportional). Throughout the winter, soil respiration was restricted to wet meadow locations, and the magnitude of over-winter soil respiration from these areas was comparable to the upper range of carbon respired by seasonally snow-covered soils. These data were corroborated by continuous eddy covariance-based net ecosystem exchange measurements, which showed that the alpine tundra lost an average of 232 g C m-2 to the atmosphere between 2008 and 2014. To test the degree to which climate change may be responsible for these carbon cycling patterns, radiocarbon analysis was used to constrain the turnover time of respired soil organic carbon, which was indicative of permafrost degradation at one wet meadow location, and could thereby signal an important feedback to climate change. In the future, the carbon source or sink strength of snow-scoured alpine tundra will depend on the seasonal magnitude of the feedbacks between changing climate, soil physical properties, and biological activity that are identified by this work.