A dynamic soil chamber system coupled with a tunable diode laser for online measurements of δ 13 C, δ 18 O, and efflux rate of soil‐respired CO 2

High frequency observations of the stable isotopic composition of CO 2 effluxes from soil have been sparse due in part to measurement challenges. We have developed an open‐system method that utilizes a flow‐through chamber coupled to a tunable diode laser ( TDL ) to quantify the rate of soil CO 2 efflux and its δ 13 C and δ 18 O values ( δ 13 C R and δ 18 O R , respectively). We tested the method first in the laboratory using an artificial soil test column and then in a semi‐arid woodland. We found that the CO 2 efflux rates of 1.2 to 7.3 µmol m −2  s −1 measured by the chamber‐ TDL system were similar to measurements made using the chamber and an infrared gas analyzer ( IRGA ) (R 2  = 0.99) and compared well with efflux rates generated from the soil test column (R 2  = 0.94). Measured δ 13 C and δ 18 O values of CO 2 efflux using the chamber‐ TDL system at 2 min intervals were not significantly different from source air values across all efflux rates after accounting for diffusive enrichment. Field measurements during drought demonstrated a strong dependency of CO 2 efflux and isotopic composition on soil water content. Addition of water to the soil beneath the chamber resulted in average changes of +6.9 µmol m −2  s −1 , −5.0‰, and −55.0‰ for soil CO 2 efflux, δ 13 C R and δ 18 O R , respectively. All three variables initiated responses within 2 min of water addition, with peak responses observed within 10 min for isotopes and 20 min for efflux. The observed δ 18 O R was more enriched than predicted from temperature‐dependent H 2 O‐CO 2 equilibration theory, similar to other recent observations of δ 18 O R from dry soils (Wingate L, Seibt U, Maseyk K, Ogee J, Almeida P, Yakir D, Pereira JS, Mencuccini M. Global Change Biol . 2008; 14: 2178). The soil chamber coupled with the TDL was found to be an effective method for capturing soil CO 2 efflux and its stable isotope composition at high temporal frequency. Published in 2010 by John Wiley & Sons, Ltd.