Effect of biochar application on freezing-thawing deformation of farmland soil during freeze-thaw cycling

Freezing-thawing deformation is a common phenomenon in seasonal frozen soil areas. Based on indoor simulation experiments, the effects of biochar application amounts (0%, 1%, 2%, and 3%) on freezing-thawing deformation characteristics and the response relationships with soil properties were studied. The results showed that (1) the essence of freezing-thawing deformation is the transformation of soil pore sizes from small to large. The biochar application reduced the transformation range and rate, significantly increased the soil total porosity, promoted thaw settlement and restrained frost heaving. In addition, with increasing numbers of freeze-thaw cycles, the soil total porosity increased with low application amounts (0% and 1%) but decreased with high application amounts (2% and 3%). (2) Freeze-thaw cycling reduced the soil aggregate stability, and the application of an appropriate amount of biochar alleviated the decreasing trend, increased the stability of aggregates, reduced the proportion of silt and clay (<0.106 mm), and inhibited soil freezing-thawing deformation. (3) With increasing biochar application amounts, upward water migration and the decrease in organic matter content caused by freeze-thaw cycling gradually weakened, and the organic matter content increased significantly, inhibiting soil freezing-thawing deformation. (4) With increasing numbers of freeze-thaw cycles, the application of biochar caused secondary collapse to occur earlier, and a significant frost heave phenomenon was observed after obvious thaw settlement of soil columns with high biochar application amounts (2% and 3%). With increasing biochar application amounts, the characteristics of soil freezing-thawing deformation changed from frost heaving to thaw settlement, and an appropriate biochar application amount inhibited soil freezing-thawing deformation. Considering the soil properties and freezing-thawing deformation characteristics, 1% was suggested as the most suitable biochar application amount for farmland black soil.