Thermal Properties of Peaty Soils: Effects of Liquid‐Phase Impedance Factor and Shrinkage

Soil thermal conductivity (λ) and heat capacity ( C ) control heat transport and the thermal environment for biogeophysical processes in the vadose zone. Accurate λ and C predictions for peaty soils with high organic contents are particularly important for assessing emissions of greenhouse gases formed during microbial activity in wetlands. In this study, we measured the λ and C at different soil‐water matric potentials on undisturbed samples for three peaty soil profiles at the Hokkaido Bibai marsh in Japan, representing a total of 10 different soil horizons. The thermal properties under air‐dried conditions, λ dry and C dry , were measured separately at changing volumetric solids contents (σ). For each sample, volume shrinkage was observed to varying degrees during the drying process. Measured λ and C increased linearly with increasing volumetric water content (θ). Applying the concept of a three‐phase mixing model and incorporating the λ–θ or C –θ and the λ dry –σ or C dry –σ relations, predictive λ and C models were developed as functions of σ and θ. The new mixing model is represented by λ = λ dry + f λ θλ w and C = C dry + f C θC w , where λ w and C w are the thermal conductivity and heat capacity of water, respectively, and f is an impedance factor that takes into account the liquid‐phase tortuosity. The new mixing model predicted literature λ–θ data on peaty and highly organic soils under variable saturation well. The probable ranges of λ and C under variable saturation were proposed based on the sensitivity analysis.