Evolution mechanism of soil hydrothermal parameters under freeze–thaw cycles: Regulatory significance of straw and biochar

Abstract

In seasonally frozen regions, soil hydrothermal parameters play an important role in the study of soil water transport and heat transfer. In this study, different initial moisture contents (W1, W2, W3), different straw and biochar applications (S1, S2, B1, B2) and untreated control groups CK were used. A laboratory simulation was designed, and the changes in total soil porosity (TP), pore distribution, soil aggregates, soil saturated hydraulic conductivity (Ksat) and soil thermal conductivity (λ) were analyzed. The results showed that freeze–thaw cycles affected soil porosity and destroyed soil aggregates, while straw and biochar applications increased total soil porosity, enhanced pore connectivity, and enhanced soil aggregate stability. Freeze–thaw cycles and straw and biochar applications increased soil Ksat, and biochar showed a stronger ability to improve soil hydraulic conductivity. After 20 freeze–thaw cycles, compared with the control group, the Ksat of treatments S1 and B1 increased on average by 11.18% and 22.03%, respectively, at different moisture contents, while the Ksat of treatments S2 and B2 decreased on average by 3.65% and 6.17%, respectively. Excessive application of the amendments reduced the soil Ksat. During the freeze–thaw cycles, the soil λ of each treatment first showed a stable trend and then decreased. However, the increase in the initial moisture content induced a significant increase in soil λ. Straw and biochar applications reduced soil λ, and the reduction was proportional to the applied amount, but the thermal insulation effect of biochar was better than that of straw. This study showed that the comprehensive effect of the B1 treatment was better than that of the other treatments for improving the soil structure, and enhancing the soil infiltration capacity and enhancing soil thermal insulation capacity. A 1% biochar application is recommended as the optimal soil amendment.