Carbon and oxygen isotope ratios of ecosystem respiration along an Oregon conifer transect: preliminary observations based on small-flask sampling

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Abstract
Isotope ratio analyses of atmospheric CO2 at natural abundance have significant potential for contributing to our understanding of photosynthetic and respiration processes in forest ecosystems. Recent advances in isotope ratio mass spectrometry allow for rapid, on-line analysis of small volumes of CO2 in air, and open new research opportunities at the ecophysiological, whole-organism, and atmospheric levels. Among the immediate applications are the carbon and oxygen isotope ratio analyses of carbon dioxide in atmospheric air. Routine analysis of carbon dioxide in air volumes of approximately 50–300 μl is accomplished by linking a commercially available, trace gas condenser and gas chromatograph to an isotope ratio mass spectrometer operated in continuous-flow mode. Samples collected in the field are stored in either gas-tight syringes or 100-ml flasks. The small sample volume required makes it possible to subsample the air in flasks for CO2 and then to sample the remaining air volume for the analysis of the isotopic composition of either methane or nitrous oxide. Reliable δ13C and δ18O values can be obtained from samples collected and stored for 1–3 days. Longer-term storage, on the order of weeks, is possible for δ13C measurements without drift in the isotope ratio signal, and should also be possible for δ18O measurements. When linked with an infrared gas analyzer, pump and flask sampling system, it is feasible to sample CO2 extensively in remote forest locations. The air-sampling system was used to measure the isotope ratios of atmospheric CO2 and to conduct a regression analysis of the relationship between these two parameters. From the regression, we calculated the δ13C of ecosystem respiration of four coniferous ecosystems along a precipitation gradient in central Oregon. The ecosystems along the coast-to-interior Oregon (OTTER) gradient are dominated by spruce–hemlock forests at the wet, coastal sites (> 200 cm precipitation annually) to juniper woodlands (20 cm precipitation) at the interior, dry end of the transect. The δ13C values of ecosystem respiration along this transect differed by only 1.3‰ (range of –25.2 to –23.9‰) during August at the peak of the summer drought. Following autumn rains in September, the δ13C of ecosystem respiration in the four stands decreased; overall the difference in the carbon isotope ratio of ecosystem respiration among sites increased to 3.9‰ (–26.8 to –22.9‰).

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