Application of ground-penetrating radar to measure near-saturation soil water content in peat soils

The presence and flux of biogenic methane-rich free phase gas that is eventually released to the atmosphere influence water content (theta) of peat soils below the water table. Small variations in gas content in peat soils at near-saturation theta could be inferred by changes in dielectric permittivity, but detailed measurements in that range of theta needed to develop calibration functions are lacking. Our experiment uses a new method for varying theta in the sample using elevated pressure to reduce the naturally occurring volumetric gas content in a manner similar to what occurs in situ under atmospheric pressure change, which relevant to understanding carbon gas cycling in peatlands. We recorded dielectric permittivity using a 1.6 MHz ground-penetrating radar antenna at multiple water contents between 0.87 and 0.95 m(3) m(-3) on four peat monoliths with varying levels of humification and with <5% gas content as is commonly observed in the field. We identified empirical equations that were linear over the range of theta investigated and optimized a dielectric mixing model for estimating theta from GPR. These results indicate that there are differences in the permittivity-theta relationships developed between peat samples and suggest that variability in dielectric relationships may be attributed to peat structure and particle orientation. The empirical relationships developed in this study confirm the utility of previously applied multiphase mixing models for estimating peat properties using methods sensitive to dielectric permittivity. This work is relevant to studies of gas content in peat and the role of peatlands in the carbon cycle.