Using soil collars

Although the ideal measurement would not alter the soil in any way, we've found that soil collars present an acceptable and necessary tradeoff between affecting the soil environment and measuring the actual gas flux. Soil collars have several advantages over direct insertion of the chamber into the soil, including:

• The disturbance effect of insertion will not affect measurements conducted several hours or days later.
• It is possible to make repeated measurements at the exact same location.

Make Your Own: You can make soil collars yourself if you can find the correct size of pipe. See Making soil collars for details.

Tips for installing and using soil collars

• Install soil collars several hours to one day before making a measurement. You can test to see if the flux has stabilized by making a measurement immediately after installing the collar, and then make subsequent measurements over time. Note, however, that the soil surface CO₂ flux depends on the time of day, and the diurnal cycle can be quite large.
• In hard or compacted soils, you may need to create a channel around the collar with a knife or trowel before insertion.
• For the larger 8" collars, you can lay a piece of wood across the collar and drive the collar with a mallet.

Insertion depth

The optimal collar height will depend upon site conditions and the length of time the collars will be used at a given site. At a minimum, the collar should be inserted into the soil to a depth that gives a solid foundation so the collar does not move when placing the chamber on the collar. As insertion depth is increased, lateral diffusion of CO₂ in the soil column below the chamber will be reduced. The advantage of this is that lateral diffusion can be a source of error in the measurement (Healy et al., 1996), but the disadvantage is that as insertion depth increases, the possibility of root shearing increases. Collars may become loose over time and should be moved if this occurs.

Collars should extend a minimum of 2 cm above the soil surface for the survey chambers, and 3 cm or more for the long-term chambers, depending on the slope of the site. Collars can extend above the soil more than 2 or 3 cm, but with greater extension there is increased shading and perturbation of air movement. Over the long term, these perturbations could result in changes of evaporation rate, soil temperature, and soil moisture.

Measuring the chamber offset

The chamber offset is used to determine the volume of air inside the soil collar, which is in turn used to calculate the total system volume. The total system volume is an important part of the flux calculation, so it should be determined as accurately as possible.

The chamber offset is measured differently for the survey and long-term chambers. With the survey chambers, the chamber offset is measured by the distance between the soil surface and the top of the soil collar.

With the long-term chambers, the chamber offset is measured by the distance between the soil surface and the upper edge of the chamber base plate.

In uneven or sloping soil conditions, measure the chamber offset at multiple points within the collar and then average these measurements to represent the true offset (see Figure 2‑6).

When setting the 10 cm survey chamber onto the collar, be careful 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). Also, 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.