Response surface methodology to optimize the experimental study of target-activated microbial curing of loess

To address the limitations of traditional microbially induced calcium carbonate precipitation (MICP) technology, including reliance on exogenous bacterial strains, high costs, and poor applicability to fine-grained loess, this study introduces environmentally friendly calcium lignosulfonate into the cement solution instead of the traditional calcium source, and optimizes concentrations of nutrient yeast extract (YE), NH4Cl, urea-calcium source through single-factor and response surface Box-Behnken design experiments. By targeting the activation of indigenous urease-producing microorganisms for loess solidification, we systematically investigate the solidification mechanisms using bioactivity monitoring, unconfined compressive strength tests, calcium carbonate quantification, scanning electron microscope (SEM), X-ray diffraction (XRD) and high-throughput sequencing. The results show that: the optimized nutrient concentrations are 1.2 g/L for YE, 125 mmol/L for NH4Cl and 0.8 mol/L for urea-calcium source; the urease activity, pH value, and viable bacterial count in the optimized group peak at 120 hours of reaction, significantly promoting calcium carbonate deposition; compared to the control group (untreated loess), the optimized group exhibits 131.42%, 194.32%, and 734.65% improvements in unconfined compressive strength, secant modulus, and calcium carbonate content, respectively; needle/rod-shaped calcium carbonate crystals formed during the reaction significantly enhance soil strength and compactness through filling-bridging-cementation effects; the relative abundance of Bacillales in the optimized group reaches 93%, with notable changes in microbial community diversity and composition. These findings provide a reference for the engineering application of targeted activation technology in loess regions.