title: A dynamic soil chamber system coupled with a tunable diode laser for online measurements of delta13C, delta18O, and efflux rate of soil-respired CO2.
abstract: 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 delta(13)C and delta(18)O values (delta(13)C(R) and delta(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 micromol 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 delta(13)C and delta(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 micromol m(-2) s(-1), -5.0 per thousand, and -55.0 per thousand for soil CO(2) efflux, delta(13)C(R) and delta(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 delta(18)O(R) was more enriched than predicted from temperature-dependent H(2)O-CO(2) equilibration theory, similar to other recent observations of delta(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.
authors.affiliation: Earth and Environmental Sciences Division, MS-J495, Los Alamos National Laboratory, Los Alamos, NM 87544, USA. hpowers@lanl.gov
authors.forename: Heath H
authors.initials: HH
authors.lastname: Powers
authors.forename: John E
authors.initials: JE
authors.lastname: Hunt
authors.forename: David T
authors.initials: DT
authors.lastname: Hanson
authors.forename: Nate G
authors.initials: NG
authors.lastname: McDowell
mesh_terms: D002245:Carbon Dioxide / Q000032:analysis*; D002247:Carbon Isotopes / Q000032:analysis; D004784:Environmental Monitoring / Q000295:instrumentation* / Q000379:methods; D004867:Equipment Design; D010103:Oxygen Isotopes / Q000032:analysis; D012987:Soil / Q000032:analysis*; D014867:Water / Q000032:analysis
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D012987:Soil; D014867:Water; D002245:Carbon Dioxide