Temporal Patterns of Subsurface CO2 in Frozen Soil.

Temporal variations of CO₂ concentrations in soil gas have been monitored continually since early November 2007 (period immediately prior to soil freezing) in southeast Minnesota (Rosemount, MN).  The soil gas concentrations were monitored in a field plot that had fall moldboard plow tillage (to 18 cm) following corn (in 2007).  The CO₂ sensors used were Vaisala CARBOCAP® Carbon Dioxide Transmitter (Series GMT220).  These sensors were installed in a customized sensor chamber.  The main purpose of the chamber was to protect the sensor.  However, it also allowed a fixed equilibration volume for all soil depths and the ability to monitor temperature (inside and outside of the chamber), to pull supplemental soil gas samples (from within the sensor chamber), monitor soil gas differential pressures, and the ability to perform a field recalibration of the sensor (if needed).  In this fashion, data quality could be monitored through periodic comparison to laboratory gas chromatography.  In addition, if significant sensor drift was detected the chamber (sensor) volume could be flushed with calibration gases to perform a field recalibration of the sensor without the need to remove the sensor from the field installation.  Sensors were installed at three depths (96, 64, and 36 cm).  Multiple sensors were installed at each depth covering multiple concentration ranges.  For example, at the 64 cm depth, both a 0-5000 ppmv and a 0-5% CO₂ sensors were installed to ensure concentration readings were within the working range of the sensors, while maximizing measurement sensitivity.  At the 96 cm depth both a 0-5% and 0-20% sensors were installed.  The surface (0 cm) and 36 cm depths only had 0-5000 ppm sensors.  All sensors performed well through the winter, despite extreme cold temperatures (air temperatures <-25^oC).  The collected data indicated that there was no detectable sealing of the surface, since there were periods of pressure driven gas exchange which were in phase with barometric pressure fluctuations as well as surface wind induced pumping during the frozen and soil covered period.  However, upon soil thawing and loss of the surface snow pack, rapid increases in CO₂ concentrations were observed.  This data provides interesting insights into the dynamics of sub-surface CO₂ transport and surface exchange from frozen soils.