Initial Setup
2-21
and then subtract the distance between the
upper edge of the chamber base plate and the
top of the soil collar to obtain the offset value.
This value should be as close to 1 cm as
possible, since the default Long-Term
chamber volume value in the software
accounts for displacement by the collar. That
displacement is estimated assuming that the
collar is 7 mm thick and protrudes 1 cm
above the chamber base plate.
Figure 2-18. Chamber offset measurement using Survey and Long-Term Chambers.
In uneven or sloping soil conditions, make measurements of the chamber offset at
multiple points within the collar (i.e., North, South, East, and West), and then
average these measurements to obtain the chamber offset that is entered into
software.
When setting the 10 cm Survey chamber onto the collar, care must be taken not to
let the bottom edge of the chamber disturb the soil within the collar (this will not
occur if the collars extend more than 2 cm above the soil surface). Care should be
taken, also, to make sure the collar is not disturbed when placing the chambers on
or around them. The chamber edge should be as close to the soil surface as
practical (within 1-2 cm) so that air flow within the chamber produces mixing near
the soil surface. A foam gasket between the bottom of the Survey chambers and
the top of the soil collar minimizes leaks between the collar and the chamber. The
Long-Term chamber mounting plate uses a rubber flange that seals this space
between the chamber and the collar.
Placement of Soil Temperature and Moisture Probes
There have been numerous reports showing a strong correlation between soil CO
2
flux and soil temperature (e.g., Hanson et al., 1993, Norman et al., 1992). Soil
temperature and moisture may vary significantly with depth (Hillel, 1982), and is
dependent on a number of site characteristics (e.g., exposure to light, shade,
wind). In general, temperature and moisture measurements should be made in
close proximity to the CO
2
flux measurement.
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