A passive sampling method for radiocarbon analysis of soil respiration using molecular sieve

Radiocarbon analysis of soil CO₂ can provide information on the age, source and turnover rate of soil organic C. We developed a new method for passively trapping respired CO₂ on molecular sieve, allowing it to be returned to the laboratory and recovered for C isotope analysis. We tested the method on a soil at a grassland site, and using a synthetic soil created to provide a contrasting isotopic signature. As with other passive sampling techniques, a small amount of fractionation of the ¹³C isotope occurs during sampling, which we have quantified, otherwise the results show that the molecular sieve traps a sufficiently large and representative sample of CO₂ for C isotope analysis. Since ¹⁴C results are routinely corrected for mass-dependent fractionation, our results show that passive sampling of soil respiration using molecular sieve offers a reliable method to collect soil-respired CO₂ for ¹⁴C analysis.