Development of chamber-based sampling technique for determination of carbon stable isotope ratio of soil respired CO2 and evaluation of influence of CO2 enrichment in chamber headspace

We developed an experimental method for precise determination of carbon stable isotope ratio (delta C-13) of soil-respired CO2 under natural condition. We devised a flask sampling system optimized for collecting soil-respired CO2 to minimize the measurement artifacts related to pressure anomaly. The delta C-13 of soil-respired CO2 was estimated from relationship between change rates of the CO2 mole fraction and the delta C-13 of the CO2 in a closed chamber at the soil surface by using two end-member simple mixing model. We tested the influence of CO2 enrichment in the soil-chamber headspace on the estimates of the delta C-13 of soil respired CO2 by using high-precision measurements of CO2 mole fraction and delta C-13. To our results, the estimates of the delta C-13 of soil respired CO2 was rather insusceptible to the influence of the CO2 enrichment in the chamber as compared with the soil CO2 efflux. Improvement of analytical precision of delta C-13 is preferred approach to reduce the error in the estimates of delta C-13 of soil respired CO2. On the other hand, extending the sampling range of CO2 mole fraction in the chamber can be cost-effective means for the error-reduction practically.