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Biogeochemistry

, Volume 95, Issue 1, pp 37–59 | Cite as

Biological and physical influences on the carbon isotope content of CO2 in a subalpine forest snowpack, Niwot Ridge, Colorado

  • D. R. BowlingEmail author
  • W. J. Massman
  • S. M. Schaeffer
  • S. P. Burns
  • R. K. Monson
  • M. W. Williams
Original Paper

Abstract

Considerable research has recently been devoted to understanding biogeochemical processes under winter snow cover, leading to enhanced appreciation of the importance of many winter ecological processes. In this study, a comprehensive investigation of the stable carbon isotope composition (δ13C) of CO2 within a high-elevation subalpine forest snowpack was conducted. Our goals were to study the δ13C of biological soil respiration under snow in winter, and to assess the relative importance of diffusion and advection (ventilation by wind) for gas transport within snow. In agreement with other studies, we found evidence of an active microbial community under a roughly 1-m deep snowpack during winter and into spring as it melted. Under-snow CO2 mole fractions were observed up to 3,500 μmol mol−1, and δ13C of CO2 varied from ~−22 to ~−8‰. The δ13C of soil respiration calculated from mixing relationships was −26 to −24‰, and although it varied in time, it was generally close to that of the bulk organic horizon (−26.0‰). Subnivean CO2 and δ13C were quite dynamic in response to changes in soil temperature, liquid water availability, and wind events. No clear biologically-induced isotopic changes were observed during periods when microbial activity and root/rhizosphere activity were expected to vary, although such changes cannot be eliminated. There was clear evidence of isotopic enrichment associated with diffusive transport as predicted by theory, but simple diffusive enrichment (4.4‰) was not observed. Instead, ventilation of the snowpack by sustained wind events in the forest canopy led to changes in the diffusively-enriched gas profile. The isotopic influence of diffusion on gases in the snowpack and litter was greatest at greater depths, due to the decreased relative contribution of advection at depth. There were highly significant correlations between the apparent isotopic content of respiration from the soil with wind speed and pressure. In summary, physical factors influencing gas transport substantially modified and potentially obscured biological factors in their effects on δ13C of CO2 within this subalpine forest snowpack.

Keywords

Carbon dioxide Microbial Niwot Ridge AmeriFlux site Soil respiration Stable isotope Winter 

Notes

Acknowledgements

Data from this project are available for collaborative use by anyone interested—contact the senior author. Kurt Chowanski, Lucas Zukiewicz, and Dave Millar of the University of Colorado Mountain Research Station and Niwot Ridge Long-Term Ecological Research (LTER) project helped maintain the TDL in the field, and these folks plus Ken Hill and Scott Jackson dug the snow pits. Thanks to Sarah Gaines and Andy Schauer for preparation and measurement of calibration gases at the Stable Isotope Ratio Facility for Environmental Research at the University of Utah, and to Chris Landry of the Center for Snow and Avalanche Studies and Bert Tanner of Campbell Scientific for helpful discussions regarding snow temperature measurement. We thank Paul Brooks, Thure Cerling, and Andrew Moyes for helpful discussions and comments on an early draft. We are grateful to the USDA Natural Resources Conservation Service, Colorado Snow Survey Program for maintaining the SNOTEL network and freely sharing their data for this study. This research was supported by a grant to DRB from the Office of Science (BER), U. S. Department of Energy, Grant No. DE-FG02-04ER63904. We acknowledge the additional support of a grant from the National Institute for Climate Change Research (NICCR) Western Section to RKM, and National Science Foundation grant DEB 0423662 to the Niwot Ridge LTER program.

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Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  • D. R. Bowling
    • 1
    Email author
  • W. J. Massman
    • 2
  • S. M. Schaeffer
    • 1
    • 3
  • S. P. Burns
    • 4
    • 5
  • R. K. Monson
    • 6
    • 7
  • M. W. Williams
    • 8
  1. 1.Department of BiologyUniversity of UtahSalt Lake CityUSA
  2. 2.Rocky Mountain Research StationUSDA Forest ServiceFort CollinsUSA
  3. 3.Department of Ecology, Evolution, and Marine BiologyUniversity of CaliforniaSanta BarbaraUSA
  4. 4.Department of Ecology and Evolutionary BiologyUniversity of ColoradoBoulderUSA
  5. 5.National Center for Atmospheric ResearchBoulderUSA
  6. 6.Department of Ecology and Evolutionary BiologyUniversity of ColoradoBoulderUSA
  7. 7.Cooperative Institute for Research in Environmental SciencesUniversity of ColoradoBoulderUSA
  8. 8.Institute of Arctic and Alpine Research and Department of GeographyUniversity of ColoradoBoulderUSA

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