LI-COR LI-8100A - Section 4. Theory of Operation; Measuring Carbon Dioxide Flux from the Soil

4 Theory of Operation
Measuring Carbon Dioxide Flux from the Soil
Carbon dioxide in the soil is produced by respiration from plant roots and
microorganisms surrounding the roots, and from heterotrophic microorganisms that
metabolize plant litter and soil organic matter. In some soils, CO
2
is also generated
by the action of rainwater on calcareous substrates. Respiration and metabolism
depend strongly on temperature, so it is not surprising to find that CO
2
efflux from
the soil is also temperature dependent.
Carbon dioxide moves from the sites of production to the atmosphere primarily by
diffusion through air-filled pores and cracks in the soil, but it can also be driven by
local changes in pressure due to wind or volumetric displacement by rain. The air-
filled porosity of the soil varies with soil type and moisture content, so these
characteristics can have a significant effect on CO
2
movement in the soil.
The LI-8100A uses the rate of increase of CO
2
in a measurement chamber to
estimate the rate at which CO
2
diffuses into free air outside the chamber. For such
an estimate to be valid, conditions must be similar inside and outside the chamber;
these conditions include the concentration gradients driving diffusion, barometric
pressure, temperature and moisture of the soil.
The CO
2
gradient between the soil surface layer and air are not exactly the same
inside and outside the chamber, because there is an increase in CO
2
mole fraction
inside the chamber. The diffusion rate is estimated and corrected for using an
analytical technique that takes into account the effects of increasing chamber CO
2
concentration on the diffusion gradient. This makes it possible to estimate the
initial rate of CO
2
increase that occurred immediately after the chamber closed.
It is also important to consider the effect of the presence of the chamber on CO
2
gradients within the soil. Detailed diffusion model studies have shown that
chambers can alter gas concentration gradients in the soil, leading to errors in CO
2
flux estimates (Healy, et al., 1996). We recommend limiting measurement times to
about 1 1/2 to 3 minutes in order to keep chamber CO
2
concentration changes as
small as possible, and minimize this effect.