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Harvard Forest Symposium Abstract 2005

  • Title: Carbon isotope ratio of nighttime respiration, leaf and soil organic matter in a temperate deciduous forest
  • Primary Author: ChunTa Lai (San Diego State University)
  • Additional Authors: Jim Ehleringer (University of Utah)
  • Abstract:

    Carbon isotope ratios of respired CO2 were measured for the aboveground canopy (d13Cc), soil efflux (d13Cs) and the whole ecosystem (d13CR) on the same night in a temperate deciduous forest. Those measurements were made twice (in May and July). Flasks were filled by pulling air from (1) a branch bag, (2) a soil chamber, and (3) multiple heights inside the canopy to measure values of d13Cc, d13Cs and d13CR, respectively. The branch bag was tested and found suitable for d13CO2 measurements by Bowling et al. (Agril. For. Meteorol. 2003, 116:159-179). Polymer tubing from a sampling protocol, which includes 6 flasks installed in parallel, a gas analyzer, a pump and a flow meter controller, was inserted with a branch of leaves into the bag. Sampling began by initially flushing flasks for 5 minutes with ambient air, and then the bag was sealed to form a close loop. CO2 concentration increased as leaf-respired CO2 started to build up. Air samples were collected by closing stopcocks on a flask as CO2 increased, at an increment of 10 ppm, until all 6 flasks were filled. Measurements were repeated for 3 different trees. Similarly, flasks were filled from a soil chamber using the same sampling protocol. Higher fluxes from soil surface allowed us to fill flasks at larger CO2 increment (20 ppm). The soil chamber was equipped with a manometer (223B, MKS instrument, MA, USA), which recorded pressure differences between the ambient and that inside the chamber. The pressure inside the chamber was slightly smaller than the ambient pressure (0.1-0.3 Pa) during our measurements (repeated 3 times from 3 different soil collars). Fang and Moncrieff (1996, Func. Ecol. 10:297-305) recommended that pressure differences to be maintained as small as possible (± 0.2 Pa) for reliable measurements of soil CO2 flux. However, the effect of pressure perturbation on measuring isotope ratios of soil CO2 efflux was not well understood. All the air samples were dried before collection by flowing air through a magnesium perchlorate trap, including those collected in canopy air for the estimate of d13CR. Flasks were analyzed for both CO2 concentration and d13C using a mass spectrometer (Delta plus XL, Finnigan, Germany). A two-source mixing model (the Keeling plot approach) was then used to estimate the d13C of respired sources (i.e. canopy versus soil) and total ecosystem respiration. Values of d13CR represent a flux-weighted d13C for the whole ecosystem, which should be presumably bound by d13C values of soil and canopy fluxes. Our measurements showed that, however, values of d13Cc (-25 ‰ with one exception at -27 ‰) and d13Cs (-25 ‰) were more positive than those of d13CR (-27 ‰). Other researchers have also observed such a lack of mass balance closure in coniferous forests (Dave Bowling, personal communication). When comparing measured d13Cc and d13Cs values to carbon stocks, d13Cc values were ~ 3 ‰ more enriched compared to leaf organic matter and d13Cs values were also more enriched relative to soil organic matter (by 1 ‰) and the litter (by 3 ‰). We suspect the lack of a d13C mass balance closure with respect to nighttime respired CO2 is an artifact associated with our bag/chamber measurements. Although we followed the recommendation of Fang and Moncrieff (1996) maintaining a small pressure difference during soil chamber measurements, we do not know quantitatively how such a pressure difference affects d13C measurements. Similar errors could also have occurred during measurements of leaf respiration. A laboratory test is undergoing to determine the effect of pressure on the measurements of d13C associated with respired CO2 fluxes.

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