<p>We applied the low-cost non-dispersive infrared sensor module K33 (ICB, Senseair, Sweden) for measurements of soil CO<sub>2</sub> concentration. We integrated the sensor module in a new soil probe suitable for in situ measurements of soil gas CO<sub>2</sub> concentration. Therefore, we covered the sensor module with epoxy resin. For continuous measurements, we connected our soil CO<sub>2</sub> probe to a microcontroller (MEGA 2560 Rev3, Arduino.cc, Italy) equipped with a data logging shield (Adalogger FeatherWing, Adafruit, USA). In a laboratory experiment, we evaluated the accuracy and precision of our soil CO<sub>2</sub> probe at changing temperature and humidity by comparison with the often used CO<sub>2</sub> probe GMP343 (Vaisala, Finland) as a reference. In a field experiment, we buried our soil CO<sub>2</sub> probe to test its performance under natural environmental conditions.</p><p>The result of the laboratory experiment is that our soil CO<sub>2</sub> probe compares well with the GMP343, even at maximum relative humidity. The accuracy (<0.1 % CO<sub>2</sub>) was below the accuracy given by the manufacturer. The field experiment demonstrated that our soil CO<sub>2</sub> probe provides high-quality measurements of soil CO<sub>2</sub> concentrations under in situ soil conditions. After retrieving it, it still measured with the same accuracy and precision as before.</p><p>In summary, we used the sensor module K33 for the first time to measure in situ soil CO<sub>2</sub> concentrations by integrating it into a newly developed probe. The cost-efficient availability of our CO<sub>2</sub> probe opens up the opportunity to carry out continuous soil CO<sub>2</sub> measurements over long time periods with simultaneously high spatial resolution.</p>

We tested the performance of a newly developed CO 2 probe, K33SOIL [based on the sensor module K33 (Senseair, Sweden)], under harsh temperature and humidity conditions in a laboratory experiment. The K33SOIL, competed well (RMSE = 0.032% CO 2) with the often used GMP343 (Vaisala, Finland) in precision and accuracy. A field experiment showed that the K33SOIL is also suitable for in situ applications. Thus, the K33SOIL is a valuable and cost-efficient tool for measurement of soil CO 2 concentration.