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
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.