Measurement of ¹⁸O/¹⁶O in the soil‐atmosphere CO₂ flux

Abstract

We demonstrate that during measurements "invasion" of atmospheric CO2 (the diffusion of ambient CO2 into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the 180 of the soil-respired CO2. The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO2 diffusion out of the soil to be 7.2 +_ 0.3%0. High-resolution (1 cm) depth profiles of 18 0 of near-surface (top 10 cm) soil CO2 were carried out by gas chromatography--isotope ratio mass spectrometry (GC-IRMS). This novel technique allowed us to observe the competitive diffusion-equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO2 18 0 content.

Long‐term record of atmospheric CO2 and stable isotopic ratios at Waliguan Observatory: Background features and possible drivers, 1991–2002

“…Long-term observation of the atmospheric CO 2 mixing ratio and d 13 C from a globally distributed in situ and discrete air sampling network can enable the determination of source/sink variability and allow quantitative partitioning of fluxes into terrestrial and oceanic reservoirs because these exchange paths influence CO 2 isotopes in different ways [Ciais et al, 1995a[Ciais et al, , 1995bFrancey et al, 1995;Nakazawa et al, 1993Nakazawa et al, , 1997aNakazawa et al, , 1997bTans et al, 1990;Trolier et al, 1996]. Additionally, d 18 O measurements allow the separation of terrestrial net ecosystem production (NEP) into its photosynthetic and respiratory components on the basis of their contrasting effects on the d 18 O of atmospheric CO 2 [Ciais et al, 1997a[Ciais et al, , 1997bFlanagan et al, 1997;Ishizawa et al, 2002;Miller et al, 1999]. The CO 2 mixing ratio and isotope measurements can also be used in studies of natural variability in the carbon cycle and in calibrating global carbon budget models [Heimann and Maier-Reimer, 1996;Keeling et al, 1989aKeeling et al, , 1989bKeeling et al, , 1995Tans et al, 1989Tans et al, , 1996.…”

Dynamics of isotopic exchange of carbon dioxide in a Tennessee deciduous forest

“…Thus CO2 associated with photosynthesizing leaves is labeled by the oxygen isotopic composition of enriched leaf water. Soil-respired CO2 approaches isotopic equilibrium with water in the upper layers of the soil profile, since diffusion through the soil pore space is slow relative to the time required for isotopic exchange [Hesterberg and Siegenthaler, 1991;Tans, 1998;Miller et al, 1999]. Soil water and leaf water usually differ substantially in isotopic composition, and so the CO2 interacting with each pool reflects this difference.…”

A field-based method for simultaneous measurements of the δ<sup>18</sup>O and δ<sup>13</sup>C of soil CO<sub>2</sub> efflux

“…Global carbon budgets using the 18 O of atmospheric CO 2 and CO 2 concentrations (Ciais et al, 1997;Ciais and Meijer, 1998;Peylin et al, 1999) have indicated that soil and plant isotopic fluxes each contribute roughly five times more to the observed temporal variability in the atmospheric δ 18 O-CO 2 than do oceanic or fossil fuel burning components (Miller et al, 1999). Because the global and regional scale carbon budgets include a flux of CO 2 from the soil to the atmosphere, the determination of the isotopic composition of soil CO 2 efflux remains a critical parameter to be measured in these budgets (Ciais and Meijer, 1998;Stern et al, 1999).…”

“…In the laboratory setting, dynamic flow-through chambers have been used to estimate the 18 O of soil CO 2 efflux (Miller et al, 1999). In the field, chambers have been used to measure the 18 O (Högberg and Ekblad, 1996;Flanagan et al, 1999;Lin et al, 1999) and the 13 C of soil respired CO 2 (Flanagan et al, 1999;Ekblad and Hogberg, 2000;Ekblad et al, 2002;Mortazavi and Chanton, 2002a;Fessenden and Ehleringer, 2003).…”

“…It is not clear if the 8.8‰ molecular diffusion fractionation factor is always fully expressed as CO 2 diffuses from the soil to the atmosphere (Miller et al, 1999). Laboratory investigations (Miller et al, 1999) and modeling efforts (Stern et al, 2001) suggest that the effective diffusional fractionation factor will vary depending on the environmental conditions. Error in the estimation of the oxygen isotopic ratio of soil respired CO 2 will impact the results of studies that use the stable isotopes of CO 2 to partition ecosystem gas exchange into its components (Yakir and Wang, 1996;Bowling et al, 2003b).…”