Using Keeling Plots to Trace δ¹³C and δ¹⁸O of CO₂ Through Processes of Heterotrophic Respiration, Diffusion and Soil Water Equilibration in Artificial C3- and C4-Grassland Soils

Author

Jennifer Chelladurai, University of South Florida

Graduation Year

2009

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Jonathan Wynn, Ph.D.

Committee Member

Mark Rains, Ph.D.

Committee Member

Mark Stewart, Ph.D.

Keywords

stable isotopes, carbon, oxygen, carbonates, fractionation

Abstract

Global carbon cycle dynamics and fluxes of CO₂ between biosphere and atmosphere have been progressed through the use of Keeling Plots. Processes that control and effect the isotopic composition of soil-respired CO₂, soil CO₂, and equilibrated soil carbonate are specifically addressed in this study through the use of Keeling Plots. Replicate grassland soil profiles containing either C3 or C4 homogenized organic matter were constructed and maintained under controlled settings to encourage the production of soil-respired CO₂ and the precipitation of pedogenic carbonate. Soil CO₂ was sampled over five months and analyzed with IRMS. Keeling Plots illustrated source CO₂ affected by mixing with atmospheric CO₂ near the surface and equilibration with ¹³C-depleted CO₂ at depth in the zone of likely carbonate precipitation. The δ¹³C Keeling Plot intercepts for the surface horizons (~ -24.7 per mil for C3 profiles and ~ -11.1 per mil for C4 profiles) follow the diffusion-production model when corrected with a constant 4.4 per mil diffusional fractionation, but the Keeling Plot intercepts for developing Bk horizons were curved towards depleted values (~ -36.2 per mil for C3 profiles and ~ -18.4 per mil for C4 profiles). This change in isotopic composition with depth deviates from the usual interpretations of Keeling Plots (steady-state, source to background diffusional mixing). δ¹³C Keeling Plot intercepts indicated evaporative enrichment in the surface horizons of C3 and C4 profiles). This study uses Keeling Plots as a measure of mixing to assess the efficacy of steady-state diffusion-production models of soil CO₂ equilibration with soil carbonate.

Scholar Commons Citation

Chelladurai, Jennifer, "Using Keeling Plots to Trace δ¹³C and δ¹⁸O of CO₂ Through Processes of Heterotrophic Respiration, Diffusion and Soil Water Equilibration in Artificial C3- and C4-Grassland Soils" (2009). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/1896