Long-Term Performance on Drought Mitigation of Soil Slope Through Bio-Approach of MICP: Evidence and Insight from Both Field and Laboratory Tests

Drought is a serious global environmental issue that causes water resource scarcity and threatens agriculture and food supplements. This study aims to investigate the long-term performance of an eco-friendly technique-microbial induced carbonate precipitation (MICP) on drought mitigation at field and laboratory scales. Seven in-situ slopes treated with different MICP rounds and cementation solution concentrations were subjected to 16-month weathering. Tests were conducted to evaluate the evaporation characteristics, water retention capacity, and CaCO₃ distribution. Laboratory soil samples were further prepared to provide evidence related to underlying weathering mechanisms. The results show that MICP has a time-dependent performance on drought mitigation. After MICP treatment, soil performs a remarkable evaporation suppression ability and the evaporation rate can decrease by 50%. This is attributed to the soluble salts which increase soil water retention capability and dense hard crust which inhibits water vapor migration into the atmosphere. However, the soluble salts and crust are sensitive to weathering thus leading to degradation of MICP. Suffering 16-month weathering, the MICP-induced CaCO₃ decreases by more than 60%. The evaporation rate of soil increases with MICP rounds and cementation solution concentrations and can reach nearly two times of untreated soil. MICP-treated field soil exhibits weaker water retention capacity than untreated soil because MICP alters soil microstructure which expands macropores and decreases volume of micropores. Connected macropores act as favorable evaporation channels and accelerate evaporation. To ensure MICP long-term effects, periodical treatments are necessary. The most effective MICP treatment scheme is four to six treatment rounds and 1.0 M cementation solution.