2.2.5. Measurement of the Oxygen Isotopic Ratio of Soil CO2 and Soil-Respired CO2
In order to better understand and eventually predict inter-annual variations in the growth rate of atmospheric carbon dioxide, the interaction between the biosphere and atmosphere must be understood in a quantitative manner. Recent advances, using the carbon isotopic ratio (d13C) of atmospheric CO2, have enabled the quantification of global Net Ecosystem Productivity (NEP), that is, the total terrestrial carbon sink. Analyzing the oxygen isotopic ratio, d18O, of atmospheric CO2 allows for the partitioning of global NEP into its two componentsŸphotosynthesis and respiration. Measuring and understanding the variations in d18O of CO2 respired from soils is critical to such an analysis. We have designed a novel dynamic gas exchange system to measure the d18O of soil CO2 in the laboratory. We have also developed a novel system to examine the vertical profile of d18O of soil CO2, in situ.
The dynamic soil gas exchange system consists of a Lucite chamber sealed to the top of a steel box containing the soil profile. Air of a known CO2 mole fraction and isotopic composition is pumped through the chamber at a well-defined flow rate and is allowed to interact with the soil. Soil-respired CO2 mixes with the input air, altering its mole fraction and isotopic ratio. Air above the soil also diffuses into the soil and isotopically equilibrates with the soil water. This "invading" CO2, now with an altered d18O, diffuses back out of the soil, further altering the d18O of the CO2 in the headspace. The mole fraction and isotopic ratios of the CO2 entering and leaving the chamber are measured, and the d18O of the soil-respired CO2 is calculated using a mass-balance model that takes the invasion effect into account. We found that the isotopic ratio of soil-respired CO2 is strongly controlled by the average isotopic composition of the water in the top 10 cm of the soil which varies greatly as a function of season (run-off versus local precipitation) or short-term drying. The d18O value of the respired CO2 is also influenced by the effective kinetic isotopic effect associated with CO2 diffusion out of the soil. This effective fractionation factor is in fact a measure of the competition between CO2 diffusing out of the soil and simultaneously equilibrating with soil water and was calculated using a soil CO2 diffusion model. The theory of the processes mentioned above was described by Tans [1998].
The in situ soil CO2 profile system is based on gas chromatography-isotope ratio mass spectrometry (GC-IRMS). Aliquots of soil air (200 ml) are rapidly evacuated with a fused silica probe from 1.0 to 10.0 cm depths. This air is dried with a Nafion membrane and passed directly to a porous polymer capillary column for separation of CO2 from air and other trace species (including N2O.) After separation the CO2 is admitted to the IRMS, via an open split, for determination of both d13C and d18O of the CO2. Precision is better than 0.1‰ for both ratios in an analysis time of only 6 minutes. Comparison of a soil CO2 isotopic profile with an isotopic profile of water from the same soil showed that CO2 and water were in full equilibrium until a depth of 4 cm, above which the CO2 became progressively isotopically enriched towards the surface. These results were nearly identical to those predicted by a model calculation on the same soil system.
Both the dynamic soil gas exchange system and the in situ soil CO2 profile system enabled us to examine the physical processes controlling the isotopic ratio of soil-respired CO2 in a controlled laboratory setting. Both methods of analysis confirmed the ability of the soil CO2 diffusion model to accurately predict the isotopic ratio of soil-respired CO2 and the CO2 within the soil given knowledge of soil characteristics and the oxygen isotopic profile of soil water.