To add to the list, there is also CO2 profile method. This method is suitable for long-term measurements (from some days to years), which this message is dealing with.
The text below that I just wrote is quite technical and surely I forgot to mention some of the key things that are necessary to understand what this is all about.
HOW TO MEASURE SOIL CO2 CONCENTRATION?
This method requires some tubes to be buried underground. The tube should be solid, but the part of the tube that is underground should be drilled with small holes, and wrapped into some material that is porous but blocks sand from going inside the tube. Now the gas concentration in the tube underground settles to the same as the gas concentration in the soil airspace. Gas can be sampled via the tube, and analyzed later. This is a very efficient way of sampling large areas quickly.
WHY TO MEASURE SOIL CO2 CONCENTRATION? | THEORY
There are two interpretations of CO2 flux (gas transport).
- CO2 flux originates from the CO2 production. If there are no storage changes (the system is in equilibrium), then:
Flux = Production (equation 1)
- CO2 flux originates from the CO2 concentration difference between the soil and the atmosphere. If we know the CO2 concentration both in the atmosphere and in the soil, and how fast it diffuses to the atmosphere, we can calculate the flux.
CO2 Flux = K * ( [CO2soi] - [CO2air] ), (equation 2)
where K is transport coefficient, [CO2soil] is CO2 concentration in soil airspace and [CO2air] is CO2 concentration in the atmosphere (about 400ppm). Using equation 1, we get the CO2 production (~=respiration).
Transport coefficient (K) varies spatially (in space). Usually it is modeled, but this takes time and requires information on the soil and some theoretical knowledge. For larger areas with several measurement points, a literature average could be used to get one average number. The funny thing is that K can be very easily measured. I think the reason why no-one in the research community does not do it, is because in theory K also varies temporally (in time). This means that it should be constantly being measured. However, my experimental results in Hyytiälä forest shows that it does not change much at all! This means that it only needs to be measured once (per year or whatever), by using chambers!
CO2 profile method is a low-cost method of estimating CO2 production of soils. The interpretation of the data may be tricky, but based on my experiments, the issues can be solved by measuring the transport coefficient. This would mean that soil CO2 production could be measured in places where it has not been done before.