Thermal Properties of the Urban Soil Upper Horizon under Application of Different Doses of Biochar

The study investigates the thermal properties of the upper arable horizon of urban soil (urbanozem) sampled in the territory of the Experimental Soil Station of Moscow State University, birch biochar, and their mixtures. Soil from Ap horizon was sieved through a 1-mm sieve and then mixed with biochar in various proportions; the biochar content in the mixtures ranged from 1 to 15% by dry mass. Plastic cylinders 5.1 cm high and 7.7 cm in diameter were filled with soil, biochar, and mixtures thereof. The bulk density of the samples varied from 1.34 g cm–3 for the Ap horizon to 0.28 g cm–3 for biochar. Thermal properties were measured with a probe method using a TEMPOS device with an SH-3 probe; the distance between the probe needles was 0.6 cm. A series of measurements were performed for each of the six samples with a stepwise change in sample moisture content from the maximum after capillary saturation to the minimum in the air-dry state. Thermal properties for each sample were measured at all moisture levels with ten replications, successively placing the probe in different positions within the central zone of the sample. The heat capacity, thermal conductivity, and thermal diffusivity of the substrates consistently decreased with increasing biochar content over the entire moisture content range; the highest values of all thermal properties were obtained for the Ap horizon, and the lowest, for biochar. All samples demonstrated a linear growth of volumetric heat capacity with moisture content, and the slopes of the obtained curves were quite similar. The specific heat capacity of soil moisture calculated from the experimental data was equal to 3406.8 J kg–1 K–1, which confirms the concept that the heat capacity of water in the soil is less than the heat capacity of free water. The thermal conductivity of biochar increased almost linearly with moisture content. The thermal conductivity of soil and mixtures increased rapidly with moisture content in the range of 0.03–0.20 cm3 cm–3, then the growth slowed down, and, with an increase in moisture above 0.45 cm3 cm–3, the thermal conductivity decreased. The thermal diffusivity of biochar was very low and almost constant over the entire moisture range. The dependences of thermal diffusivity versus moisture content for soil and mixtures were similar to the curves for thermal conductivity, but had two weakly expressed maxima, the first of which was observed at a moisture content below 0.20 cm3 cm–3. It is concluded that the introduction of biochar into loamy soil reduces its thermal diffusivity and thereby stabilizes the soil temperature regime, reducing the range of diurnal temperature fluctuations and slowing down seasonal warming. For urban conditions with high summer temperatures, this effect can be regarded as positive; for regions with low heat supply, the introduction of biochar into the soil is rather undesirable, at least in the context of soil temperature regime.