A Sensor Probe with Active and Passive Humidity Management for In Situ Soil CO 2 Monitoring.

Soil CO ₂ concentration and flux measurements are important in diverse fields, including geoscience, climate science, soil ecology, and agriculture. However, practitioners in these fields face difficulties with existing soil CO ₂ gas probes, which have had problems with high costs and frequent failures when deployed. Confronted with a recent research project's need for long-term in-soil CO ₂ monitoring at a large number of sites in harsh environmental conditions, we developed our own CO ₂ logging system to reduce expense and avoid the expected failures of commercial instruments. Our newly developed soil probes overcome the central challenge of soil gas probes-surviving continuous exposure to soil moisture while remaining open to soil gases-via three approaches: a 3D printed housing (economical for small-scale production) following design principles that correct the usual water permeability flaw of 3D printed materials; passive moisture protection via a hydrophobic, CO ₂ -permeable PTFE membrane; and active moisture protection via a low-power micro-dehumidifier. Our CO ₂ instrumentation performed well and yielded a high-quality dataset that includes signals related to a prescribed fire as well as seasonal and diel cycles. We expect our technology to support underground CO ₂ monitoring in fields where it is already practiced and stimulate its expansion into diverse new fields.