HR: 1340h
AN: B13B-1185    [Abstracts]
TI: Using Headspace Equilibration to Measure the d13C of Soil-Respired CO2
AU: * Robertson, M A
EM: robertsonm01@gmail.com
AF: Carleton College, 300 N. College Street, Northfield, MN 55057, United States
AU: Powers, E
AF: University of Idaho, Dept of Forest Resources, Moscow, ID 83844-1133, United States
AU: Marshall, J
AF: University of Idaho, Dept of Forest Resources, Moscow, ID 83844-1133, United States
AB: Soil respiration is an important component of the global carbon cycle and can account for as much as 70% of ecosystem respiration. Soil gas flux measurements have been combined with stable isotope analysis to examine ecosystem properties and processes such as water-use efficiency and the role of above ground weather in controlling soil respiration. However, current methods of measuring the δ¹³C of soil-respired CO₂ are either inherently inaccurate or time-consuming and tedious. An alternative method of obtaining this value offers a potential solution to these problems. In this method, plastic chambers are fitted with rubber septa to allow for sample collection, then inverted and partially buried in soil. The chamber headspace is allowed to come to equilibrium with soil air. In this study we tested the viability of this method by examining whether frequent resampling of respiration chambers affected δ¹³C measurements, whether headspace CO₂ concentration and δ¹³C values approached equilibrium asymptotically, and whether simulated and actual diel temperature cycles affected estimates of δ¹³C. All experiments were conducted on respiration chambers inverted in potting soil and placed in a Conviron growth chamber, with the exception of one field test that was conducted on respiration chambers installed in a Northern Idaho experimental forest. Samples were collected with a syringe and stored in glass vials for analysis by a ratioing mass spectrometer. We found that resampling respiration chambers as frequently as every 10 minutes had no significant effect on final δ¹³C values, that both chamber CO₂ concentrations and δ¹³C values exhibited an asymptotic approach to equilibrium, and that the equilibrium value was offset from the initial flux by the amount we expected, approximately 4 ‰. However, we also found that diel temperature variation affected both headspace CO₂ concentration and δ¹³C in the lab and in the field. We concluded that if this method is used in areas with relatively slight diel temperature fluctuation, it provides a viable alternative to current methods of measuring the δ¹³C of soil respiration.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4870 Stable isotopes (0454, 1041)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting