By Jenny Soong
Soils respire CO2 through the decomposition of soil organic matter and root activity. That CO2 movement from the soil to the atmosphere is an important part of the global carbon cycle, which keeps life on earth in balance and regulates our climate.
Measuring soil respiration in the field isn’t too difficult, but it does require some careful consideration of soil processes, and how your measurements may interfere with them. The basic concept is this: CO2 is produced in the soil through decomposition and root respiration. Therefore, there is a high concentration of CO2 in the soil, which diffuses out through the tortuous soil pore pathways, and into the atmosphere. What you’re really measuring is that diffusion, or flux rate.
Here are just a few tips of the trade I’ve picked up over the past four years of using dynamic soil chamber measurements to measure soil respiration…
Many different types of soil chambers can be used to measure soil respiration. I have the most experience with the Licor LI-8100. It’s my workhorse. It’s probably the top-of-the-line gadget for soil respiration geeks like me. It has an automated chamber connected to a field-sturdy infrared gas analyzer (IRGA) that measures CO2 concentrations continuously in the field. The main reason it trumps other analyzers is because of its pressure-equilibrating saucer on top and a small internal mixing fan. These two things help to ensure that you aren’t changing the pressure conditions inside the chamber, which could alter the diffusion of CO2 out of the soil.
2) Use collars!
Soil collars are short sections of PVC plumbing pipe that are beveled at one end to slice into the soil to create an airtight seal between the soil and your chamber. Without the soil collar, CO2 from under your chamber could easily leak out the sides of your plot causing an underestimation of the CO2 flux. Install your soil collars at least 24 hours before taking your soil respiration measurements to allow the soil to settle back in around the collar (the initial disturbance of the soil can cause CO2 fluctuations). I typically install my collars to 2-5 cm deep depending on the soil system I’m working in.
3) The dead band
The dead band occurs in the first few seconds after the chamber closes down on the collar. The closing of the chamber causes an initial pressure disturbance to your system, which disturbs the CO2 flux rate. I recommend throwing out those first few seconds of data and using the flux rate estimate after the diffusion rate re-equilibrates.
You respire CO2 too! Take care when taking chamber measurements to not breathe directly into your chamber as it is closing. I check for this by taking note of the atmospheric concentration of CO2 and making sure that all of my readings start at about that level.
5) Air tight seal
The whole point of installing the collar is to ensure that there is an airtight seal between the chamber and the soil. This ensures that you capture all of the CO2 diffusing out of the soil for an accurate soil respiration measurement. When you move your soil chamber from collar to collar, make sure that it fits tightly onto the collars for each measurement.
6) Length of chamber closure
The LI-8100 is a closed system, meaning that that the CO2 that is produced in the system while the chamber is closed accumulates in the chamber. Make sure that your chamber is not closed too long, which causes a large increase in the CO2 concentration within the chamber and could affect the rate of CO2 diffusion from the soil. I typically use 2 minute chamber observations.
7) Other types of chambers and analyzers
Many other types of chambers and soil respiration systems exist, other than the LI-8100. Papers have been written about the pros and cons of these systems. You can even build your own soil respiration chambers out of salad bowls and salad saucers (ask Dr. Joe von Fischer)! Laser spectroscopy is a new alternative to IRGAs in the field (check out the Picarro and Los Gatos systems). However, always remember to take into consideration the disturbance and pressure differentials that may affect the diffusion gradient you’re measuring.
This was just a very brief description of some key points I have learned about taking chamber based soil respiration measurements over the past few years, and a huge body of scientific literature exists on the topic (i.e. here, here, here, here, here, here, here). Please feel free to ask any questions or share more insight!