Arable ecosystem – source or sink for carbon?

At the Oensingen research station, the CO₂ exchange between the field and the atmosphere has been measured continuously since December 2003. When, how much and under what conditions the field releases CO₂ into the atmosphere is important to understand, as CO₂ is the main cause of climate change.

When wheat grows in the field, photosynthesis takes place during the day and CO₂ is absorbed from the atmosphere, most of which is bound in the leaves and roots. At night, the process is reversed and, due to respiration, CO₂ is released into the atmosphere. After the wheat has been harvested, there is no more CO₂ uptake and there is only CO₂ release into the atmosphere until the next crop grows.

The CO₂ exchange is measured using the eddy covariance method. Two measuring devices are needed for this: the ultrasonic anemometer for horizontal and vertical wind speed and the gas analyser for determining the CO₂ concentration. Measurements are taken 20 times per second, which corresponds to 1,728,000 measurements per day!

At the Oensingen research station, not only are greenhouse gases continuously measured, but numerous other measured variables such as air temperature, radiation or precipitation are also collected. Combining the measured data with the harvest samples, it is possible to determine whether the field is a carbon sink or a carbon source.

The measurements accumulated since 2003 show that no crop was able to absorb enough CO₂ for the field to be a carbon sink in the long term. The main reason for this is that most of the captured CO₂ is taken back from the field via the harvest. Even the application of carbon in the form of manure or slurry could not sufficiently compensate for this.