Chitosan-Modified Biochar for Improving Water Retention in Karst Quarries: A Potential Solution for Soil Remediation

Biochar has been widely applied in soil remediation. However, few studies have been conducted on its effect on soil water retention in abandoned quarries. Moreover, due to the poor water storage capacity of the quarry, the adhesion and water retention capacity of biochar are limited in its application. Here, we used sugarcane bagasse (SB) and chicken manure (CM) prepared at 300 °C and 500 °C, and modified them with chitosan (CS) to improve the water absorption, and further explored their effects on the soil water retention characteristics in karst, abandoned quarry. The results indicated that the modified biochar significantly improves the hydrophilicity and water absorption capacity of the biochar. The water absorption multiples of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS were 131.03, 94.47, 86.19, and 114.70 g·g−1. After being applied to the quarry soil, it significantly improved the water retention characteristics. In addition, the application of modified biochar significantly increased the mean weight diameter (MWD), geometric mean diameter (GMD), and cation exchange capacity (CEC) of soil aggregates. Compared with the control, GMD of 300SBB-CS, 500SBB-CS, 300CMB-CS, and 500CMB-CS increased by 24.42%, 32.74%, 8.34%, and 21.20%, respectively. The modified biochar improves the soil’s water retention characteristics by enhancing its water absorption capacity. In addition, the modified biochar improves the stability of soil aggregates by increasing the soil CEC, which indirectly enhances the water retention characteristics of the soil. These findings provide substantial reference information for improving soil conditions in karst regions.