How do we actually measure gas (carbon dioxide, water) flux for a plant, an animal, or an ecosystem? The answer relies on something called an IRGA, or an infrared gas analyzer. Different molecules absorb certain wavelengths of light. That means that if we shine the right kind of light on some molecule, we can measure how much of the light gets absorbed by the molecules, and how much gets transmitted. The amount of light absorbed is proportional to how much of the molecule there is. For carbon dioxide, the right kind of light is in the infrared part of the spectrum, invisible to the eye. That means our machine uses an infrared laser to generate the right kind of light. You can see the apparatus in the setup above – the tall white thing is the path that the laser beam traverses, so we are measuring the atmospheric CO2 concentration.
To measure how the amount of carbon an individual leaf or section of soil is using, we have to work a little harder. Here we need to isolate the system of interest. That’s the PVC ring that couples onto a gasket we’ve installed on the desert floor. We also feed in gas with a known concentration of CO2 and H2O, through the transparent white tubes. We let the gas mix with the system of interest – maybe a leaf, maybe a tree, maybe some dirt – then send it back out through other tubes, and use another IRGA (with laser) to measure the final CO2 and H2O concentration. We can subtract these two numbers to calculate how much carbon dioxide and water was being used, or given off, by our system.
Actually doing this work is pretty easy, once our machine is set up and calibrated. The hardest part is shivering through a cold night or morning while we take measurements!