Shrinkage characteristics of lime concretion black soil as affected by biochar amendment

Studies have reported that biochar is a sustainable amendment that improves the chemical and physical properties of soil. In this study, an incubation experiment was conducted to investigate the effects of different application rates of biochar on the cracking pattern and shrinkage characteristics of lime concretion black soil after three wetting and drying cycles. Biochar derived from the corn straw and peanut shell mixture was applied to the soil at rates of 0, 50, 100, and 150 g kg−1 dry weight, representing the treatments T0, T50, T100, and T150, respectively. During the wetting and drying cycles, the cracking pattern and shrinkage characteristics of the unamended and amended soil samples were recorded. Application of biochar significantly increased soil organic carbon content in the samples. During soil desiccation, biochar significantly reduced the rate of water loss. Cracks propagated slowly and stopped due to the relatively higher water content in the soil applied with biochar. The cracking area density (ρc), equivalent width, fractal dimension, and cracking connectivity index decreased during the drying process with increasing application rate of biochar. The ρc value of the T50, T100, and T150 treatments decreased by 33.6%, 52.1%, and 56.9%, respectively, after three wetting and drying cycles, whereas the T0 treatment exhibited a marginal change. The coefficient of linear extensibility, an index used to describe onedimentional shrinkage, of the unamended soil sample (T0) was approximately 0.23. Application of 100 and 150 g kg−1 biochar to the soil significantly reduced the shrinkage capacity by 41.45% and 45.54%, respectively. The slope of the shrinkage characteristics curve, which indicates the ralationship between soil void ratio and moisture ratio, decreased with increase in the application rate of biochar. Furthermore, compared with the T0 treatment, the proportional shrinkage zone of the shrinkage characteristic curve of the T50, T100, and T150 treatments decreased by 5.8%, 13.1%, and 12.1%, respectively. Differences were not observed in the moisture ratio at the maximum curvature of the shrinkage characteristic curve among the treatments. The results indicate that biochar can alter the cracking pattern and shrinkage characteristics of lime concretion black soil. However, the effects of biochar on the shrinkage of lime concretion black soil are dependent on the number of wetting and drying cycles.