Development of a CO2 gas analyzer for monitoring soil CO2 concentrations

Measurement of soil CO2 concentrations is important for investigating the dynamics and diffusion of CO2 in soil. In this study, we developed a small CO2 analyzer for measuring in situ-soil CO2 concentrations. The CO2 analyzer consists of a module containing an infrared CO2 gas sensor, a temperature sensor, and a relative humidity sensor. These sensors are installed in a protective box with an air vent, which is suitable for burying in the soil. The output response time of the CO2 analyzer was 349 s, as evaluated from the phase lag after input of known CO2 concentrations. This response time is short enough to measure soil CO2 concentrations, because variations in concentration are slower than the response time of the analyzer. In a field test, we used the CO2 analyzer to measure soil CO2 concentrations at five depths (0-50 cm) over 2.5 months. While the CO2 concentration generally increased with depth, the amplitude of the variation in CO2 concentration decreased with depth. The phase lag of the variations in soil CO2 concentration also increased with depth, as did soil temperature. The tests confirm that the CO2 analyzer is applicable to continuous monitoring of soil CO2 concentrations.