Is your seal affecting your chamber closure and flux measurements?

Is your chamber seal affecting your flux measurements?

Manual and automated chambers provide invaluable data about greenhouse gas fluxes that can be used for applications ranging from understanding basic biological processes to evaluating feedbacks and impacts of climate change.

As greenhouse gas analyzers become more precise and offer highly resolved temporal data, methods used for chamber measurements must evolve as well. Sealing the chamber on to the soil collar to ensure a leak-free measurement has been the topic of some research (see references) and several types of seals have been developed, including piston seal chambers, face seal chambers, and water seal chambers, illustrated in the diagram below.

chamber-closure-seal-gif

Showing piston seal (left), face seal (centre), and water seal (right) closing mechanisms.

While these systems may all provide a good seal once the chamber is deployed, it is important to think about what happens to the soil-air continuum during and after deployment as this will ultimately affect the measured gas flux.

At Eosense, we designed our chamber with a face seal gasket because it is ultimately more manageable from a mechanical point of view. But the face seal design also offers benefits to the measurement of gas fluxes, that are often overlooked in chamber system designs. Shown below is a time series of differential pressure inside a soil chamber using piston seal chamber (6” vented PVC lid on a 6” PVC collar) versus a face seal chamber (eosAC).

Chamber-closure-Pressure-Data-eosAC

In this plot, positive pressure spikes represent the overpressure of the chamber and thus a likely flow of gas into the soil pore space. Negative spikes indicate suction, and air is likely being drawn from the soil pore space into the chamber or atmosphere. What is clear, and perhaps not surprising, is the piston seal system demonstrates a positive pressure spike when being deployed on the collar and a negative spike when being removed from the collar. These pressure perturbations are driven by the compression of the air parcel under the chamber as the chamber lid slides over the chamber collar. Also shown is the pressure impulse, essentially the integral of the pressure perturbation, which is a metric that combines the magnitude and persistence of a pressure spike.

The good news is that the pressure pulse is short lived (often between 5 to 10 seconds). The bad news is that while the pressure is short lived, a quick calculation using the pressure impulse shows that we displace between ~100 to 500 mL of air at STP during each chamber deployment. Assuming a 6-inch diameter collar, this means a perturbation of the first 1 – 10 cm of soil air below the chamber (depending on air filled pore space, calculated here from 25% – 50%). For many gas species the diffusive recovery from this type of perturbation could be from 10’s of minutes to hours, resulting in a very likely bias (probably an underestimate) in chamber measurements of soil gas flux.

In contrast the face seal eosAC chamber displayed no perturbation, within the limits of the differential pressure sensor (+/- 5 Pa resolution). While these results are far from conclusive, and there are certainly ways to mitigate these large pressure differentials, this data suggests that the sealing method could be one of the most important aspects of soil flux chamber design and deployment. Learn more about the eosAC design in the blog post “Science behind the eosAC”.

 

More on chamber sealing in these papers:

Davidson, E.A., Savage, K., Verchot, L.V., Navarro, R., 2002. Minimizing artifacts and biases in chamber-based measurements of soil respiration. Agric. For. Meteorol. 113, 21-37.

Hutchinson, G.L., Livingston, G.P., 2001. Vents and seals in non-steady-state chambers used for measuring gas exchange between soil and the atmosphere. Eur. J. Soil Sci. 52, 675–682.