Effects of biochar particle size, biochar application rate, and moisture content on thermal properties of an unsaturated sandy loam soil

Although biochar is considered as a potential soil amendment, the effect of its rates and particle sizes on soil properties such as soil thermal properties, especially in a light-textured soil has not been widely discussed. Due to the importance of soil thermal properties in many engineering, climatological and agricultural applications, this study aims to investigate and model the effects of biochar particle size, biochar rate, and soil moisture content on the thermal properties of a sandy loam soil through column experiments. The experiments were conducted using three biochar application rates (0 (as control), 1%, and 5% w/w), three biochar particle diameters (<0.15, 0.15-1, and 1-2 mm), and three water saturation (Sw) levels (0.07, 0.2, and 0.78) in three replicates. Given the experimental data, soil thermal properties were modeled using the Campbell, Zhao et al., and de Vries models. According to the results, a higher biochar application rate (i.e. 5%) significantly (P < 0.05) altered soil physical and chemical properties, i.e. bulk density, porosity, EC and pH, and soil thermal properties. Biochar particle sizes had a remarkable effect on soil thermal properties. Adding 5% biochar with particle sizes of < 0.15, 0.15-1 and 1-2 mm decreased soil thermal conductivity to 21%, 26%, and 32% at Sw= 0.07, respectively indicating a lower reduction of thermal conductivity when biochar with a smaller particle size (<0.15 mm) is applied. Applying 5% biochar of the smallest size decreased soil thermal conductivity and diffusivity, and heat capacity by 21%, 17%, and 9%, respectively, and increased thermal resistivity by 27% at Sw= 0.07. The effect of soil moisture on thermal properties (especially thermal conductivity and resistivity) was more robust than biochar amendment. Experimental modeling showed a linear relationship between soil moisture and thermal conductivity, diffusivity and heat capacity, and a power law relationship between soil moisture and thermal resistivity. Mathematical modeling showed the Campbell, de Vries, and Campbell/de Vries models perform the best in estimating thermal conductivity, heat capacity, and thermal diffusivity, respectively. The findings of this research could be used for controlling and predicting soil temperature, improving irrigating scheduling, and optimizing the quantity of biochar amendment required in agricultural fields.