>Hello,
> I am interested in CO2 recycling in the forest. Now I am trying to use
>18CO2 and 13CO2  of forest air. I would like to estimate the re-absorption
>of CO2 emitted from soil.
>Please teach me how to collect of CO2 emitted from soil without any
>fractionation or contamination during the collection. Soil chamber is OK?
> Any suggestions would be greatfully appreciated.

Hi

We have taken a few different approaches to collecting respired CO2 for
carbon and oxygen isotope analysis. We have used a Li-Cor 6200 soil
respiration system modified to include a large chamber that attaches to
collars installed in the soil. The total volume of our chamber and collar
is approx. 100 liters. By attaching the chamber to the soil collar we
create a closed space that collects CO2 released from the soil surface, the
change in CO2 concentration is monitored by an infrared gas analyzer. We
collect 100 ml subsamples of air from the chamber headspace using evacuated
pyrex flasks with high vacuum stopcocks, the flasks get attached to the
chamber via a side port. Air from the chamber headspace is passed through a
column of magnesium perchlorate before entering the sample flask. We
collect a series of approximately 10 air samples from the chamber head
space as the CO2 concentration rises over the course of approximately 15-20
minutes. We have found that during this time interval the change in CO2
concentration in the chamber remains linear. The CO2 from the 100 ml air
samples is purified and analyzed using normal mass spec techniques. The
data (sample CO2 concentration and isotopic ratio) are used to create a
mixing line, and the isotope ratio of the respired CO2 is calculated from
the intercept of the mixing line. This is basically the same approach
Keeling used in the early 1960's to collected respired CO2 from an entire
ecosystem. Some references and a description of another approach to collect
soil respired CO2 for isotopic analysis can be found in:

	Flanagan, L.B. et al. (1996) Global Biogeochemical Cycles 10: 629-640.