Co-application of biochar and biogas slurry with partial chemical fertilizer: Impacts on growth, quality, soil properties and rhizosphere microbiota of Astragalus membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao

The combined application of biochar and biogas slurry represents a promising strategy for sustainable agriculture. This study investigated the effects of co-application of biochar and biogas slurry with partial chemical fertilizer on the growth and medicinal quality of Astragalus membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao. Nine treatments were established in total. Under equivalent nitrogen, phosphorus, potassium, and irrigation conditions, four representative treatments were compared: CF (conventional fertilization), T0 (partial chemical fertilizer + 80% biogas slurry), T6 (partial chemical fertilizer + 80% biogas slurry), and T3 (partial chemical fertilizer + 80% biogas slurry). Pot experiments were conducted to evaluate the agronomic and rhizosphere responses to the combined amendment. The T3 treatment significantly increased root biomass during the medicinal root maturation stage, with increases of 10.39%, 9.81%, and 4.77% relative to CF, T0, and T6, respectively. T3 also significantly increased the concentrations of key active compounds, including astragaloside IV and calycosin, by 14.17%–30.01%. In addition, T3 improved soil nutrient status, root physiological traits, and the abundance of beneficial rhizosphere microorganisms, particularly Actinobacteriota and Ascomycota. Redundancy analysis revealed significant associations between microbial community composition and key soil and root indicators. Combined Mantel, Spearman, and random forest analyses further demonstrated that T3 exhibited the strongest coordinated soil–root physiological–microbial response and the greatest number of significant positive correlations among biomass, active compounds, and soil, physiological, and microbial variables, especially those involving microbial taxa. These findings suggest that the co-application of biochar and biogas slurry more effectively promotes biomass accumulation and active compound production by strengthening synergistic interactions within the soil–plant–microbe system. Overall, co-application of biochar and biogas slurry with partial chemical fertilizer provides a sustainable fertilization strategy for the cultivation of A . membranaceus (Fisch.) Bge. var. mongholicus (Bge.) Hsiao.