Synergistic mechanisms of biochar and fly ash on water retention and crack suppression in molybdenum tailings soil under wet–dry cycles

• Water holding capacity plays an important role in soil remediation of molybdenum tailings • The porous structure of biochar can enhance the water holding capacity of molybdenum tailings soil • Fly ash can inhibit soil cracking and improve the water storage capacity of molybdenum tailings soil • There is a significant synergistic effect between biochar and fly ash in soil under drought and rainfall cycling conditions Molybdenum tailings soil lacks organic matter and biological activity, making it highly prone to compaction and unsuitable for plant survival. Drought and heavy rainfall events further exacerbate its degradation. To improve soil quality, fly ash and biochar were incorporated and subjected to three years of natural field curing without artificial intervention. To investigate the synergistic effects of fly ash and biochar on soil remediation, indoor simulations of five consecutive wet–dry cycles were conducted. Image processing techniques and fractal dimension analysis were employed to quantitatively assess changes in soil water retention and crack development. Results indicate that the combined application of fly ash and biochar significantly suppressed soil crack formation and expansion, reducing crack density and fractal dimension while enhancing soil water-holding capacity. The treatment with 10% fly ash and 4% biochar showed the greatest improvement, increasing residual moisture content by 105.64% and reducing fractal dimension and crack rate by 28.43% and 41.75%, respectively, after the first cycle compared to the control. This mechanism primarily arises from the combined effects of biochar pore adsorption and fly ash hydration cementation, which jointly enhance aggregate stability, block rapid water evaporation pathways, and mitigate structural degradation during wet–dry cycles. This study provides the first integrated assessment of biochar–fly ash synergy in molybdenum tailings soil using fractal crack analysis. Larger-scale statistical validation and field trials are needed to confirm practical feasibility.