A dynamic soil chamber system coupled with a tunable diode laser for online measurements of δ13C, δ18O, and efflux rate of soil‐respired CO2

Abstract

AbstractHigh frequency observations of the stable isotopic composition of CO2 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 CO2 efflux and its δ13C and δ18O values (δ13CR and δ18OR, 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 CO2 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) (R2 = 0.99) and compared well with efflux rates generated from the soil test column (R2 = 0.94). Measured δ13C and δ18O values of CO2 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 CO2 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 CO2 efflux, δ13CR and δ18OR, 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 δ18OR was more enriched than predicted from temperature‐dependent H2O‐CO2 equilibration theory, similar to other recent observations of δ18OR 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 CO2 efflux and its stable isotope composition at high temporal frequency. Published in 2010 by John Wiley & Sons, Ltd.