Experimental and Analytical Methods

Flask samples of canopy air from a variety of ecosystems were generally taken from different heights within the canopy at different times during the day and the night. These measurements represent a wide range of canopy CO, concentrations ([C02]). Dried canopy air was drawn through glass flasks, [C02] were measured before the flasks were closed, and flasks were brought to the lab for further analyses (For more details on how to sample and how to prepare the air samples for mass spectrometric analyses, see Buchmann et al, 1998.) Carbon isotope ratios of canopy air (<5l3Ca>nopy) were measured using an isotope ratio mass spectrometer (IRMS, precision between 0.03 and 0.3%o, depending on the IRMS used). Carbon isotope ratios (8I3C) were calculated as

where Rsamp¡e and Rstanjar(j are the ,3C/I2C ratios of the sample and the standard (PeeDee Belemnite), respectively (Farquhar et al., 1989).

The S'3C of respired CO, during ecosystem respiration (S13CER) should represent a weighted average of all respiration processes within the ecosystem. The so-called "Keeling plot" method is based on measurements of [C02] and 5,3C in air and can be used to estimate c)l3CER. If the inverse [C02]canopy are plotted against their corresponding 8l3Camopy values (so-called "Keeling plot"), a linear relationship is obtained. This relationship reflects the mixing of tropospheric C02 with an additional C02 source that is depleted in l3C compared to the troposphere. The following linear equation describes the mixing model adequately (Keeling, 1961a, b):

S'^nopy = ^02]'ro'' (g'3Clrop - S'3CER) + SI3Cer. (2)

The intercept of this equation has been used to identify the carbon isotope ratio of the additional C02 source, e.g., in forest canopies; of ecosystem respiration (SI 3CER). This method has several advantages over scaling results from small-scale enclosure studies (e.g., of foliage, stem, and soil respiration) to estimate ecosystem respiration (Lavigne et al, 1997). The Keeling plot method integrates spatially over all autotrophic and heterotrophic respiration fluxes within the ecosystem. Furthermore, it results in a flux-weigh ted estimate of SI3CER that includes plant respiration as well as fast and slowly decomposing carbon pools and their carbon isotopic signatures.

Ecosystem carbon-isotope discrimination at, (Buchmann et al, 1998) was calculated using the 8' 1CER ratio, the y-intercept of the regression equation (Eq. 3) and the corresponding tropospheric 5l3C ratio (8,3Clrop) as

Tropospheric §13C ratios were obtained from international networks such as the NOAA/CMDL Cooperative Flask Sampling Network (National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory) in cooperation with INSTAAR (Stable Isotope Laboratory at the Institute of Arctic and Alpine Research), the CSIRO network (Commonwealth Scientific and Industrial Research Organization) or the SIO network (Scripps Institution of Oceanography). Within these networks, tropospheric air samples, collected generally in remote areas, are analyzed for [C02] (all sites) and <5]3C ratios (at selected sites; Trolier et al, 1996).

Two major sources of error should be considered for the ae estimates obtained from field measurements, errors associated with the 8I3C of tropospheric CO, and the 8I3C of respired C02. The precision of the tropospheric background data, e.g., collected by NOAA/CMDL is <0.5 ppm for [CO,], and ± 0.03%o for <5]3C (Conway et al, 1994; Trolier et al, 1996). The larger error is associated with the estimates of <5I3CER due to the nature of regression analysis (extrapolating to the y-intercept). Summarizing 49 "Keel-ing-plot" analyses, the standard error for <5I3C of respired C02 averaged 0.98%o (Buchmann etal, 1998).

0 0

Post a comment