Document: ELI-3-59-35

Isotopic equilibrium between soil CO₂ and soil water.

SULZMAN, E.W.* ^1,2, E.F.KELLY ², D.S.SCHIMEL ^1,4, E.PENDALL ³ and W.A.BRAND ¹

Max-Planck-Institut für Biogeochemie, Jena 07745 Germany ¹
Colorado State University, Fort Collins, CO 80523 USA ²
University of Colorado at Boulder, Boulder, CO 80309 USA ³
National Center for Atmospheric Research, Boulder, CO 80307 USA ⁴

Abstract:
A field study was undertaken to examine the factors influencing the isotopic exchange of CO₂ between soil and atmosphere. Sites along two elevational transects including alpine tundra, lodgepole pine, and shortgrass steppe ecosystems were sampled over two growing seasons. We developed new analytical methodology that incorporates direct equilibration of the original soil samples with CO₂ in helium, followed by irm-GCMS analysis of the ¹⁸O. The oxygen isotopic signature of soil CO₂ is set by the soil water with which it is in contact according to: H₂¹⁸O(l) + CO₂(g) MISSING CHARACTER ENTITY: harr H₂O(1) + CO¹⁸O. Previous studies assumed equilibrium to be at the soil surface but a recent laboratory study showed that the actual depth of equilibrium was between 5 and 15 cm. This field study shows that ecosystem type, and in some cases soil texture, influence the depth of equilibrium. In the shortgrass steppe ecosystem there was often disequilibrium between soil CO₂ and soil water, presumably because of high atmospheric invasion and rapid diffusion under arid conditions. In the alpine system equilibrium between water and CO₂ was close to the soil surface (< 5-10 cm), whereas it was deeper in the soil profile (> 8 cm) at the lodgepole sites. Model results suggest that the isotopic composition of soil-respired CO₂ is close to that of near-surface soil CO₂, even when there was no equilibrium between CO₂ and soil water at this depth.

Keywords: isotope, oxygen, soil water, soil CO₂

This abstract is being presented at: 11:30 AM in session:
Oral Session #71: Soil Microbial Biomass and Soil Respiration.