Effects of returning biogas slurry to fields on the growth and quality of Astragalus membranaceus var. mongholicus

A by-product of anaerobic fermentation, biogas slurry is a high-quality organic fertilizer that can be applied to farmland as a substitute for chemical fertilizers or in combination with chemical fertilizers to improve the yield and quality of crop plants. In this study, the effects of biogas slurry concentration (A₁: 100%, A₂: 80%, A₃: 50%) on the growth, physiological characteristics, yield, and quality of Astragalus membranaceus var. mongholicus were investigated by comparing the effects of BS at different application methods (B₁: spraying, B₂: topdressing, B₃: spraying + topdressing). Water and fertilizer treatments were used as controls. The results revealed that all assessed indicators of the A. membranaceus var. mongholicus plants subjected to the 11 different treatments showed differing trends, with most of the indices showing initial increases and subsequent declines during the entire growth cycle. In response to spraying biogas slurry alone, the 80% biogas slurry treatment was found to be the most effective, and was significantly superior to that of the chemical fertilizer treatment (P < 0.05). Following topdressing, compared with the other 10 treatments, the 80% biogas slurry topdressing treatment was found to promote significant increases in the chlorophyll contents, activities of antioxidant enzymes, and contents of osmoregulatory substances in A. membranaceus var. mongholicus seedlings when measured during the harvest period, as well as significant increases in the yields of the aboveground and belowground biomasses. Furthermore, in response to the 80% biogas slurry topdressing treatment, we detected significant increases of 87.09%, 60.65%, 52.17%, and 24.21% in the contents of calycosin-7-O-β-d-glycosides, ononin, calycosin, and astragaloside Ⅳ, respectively, compared with the fertilizer treatment (P < 0.05). The comprehensive evaluation concluded that 80% biogas slurry topdressing was more effective. These findings indicated that the appropriate concentration and application method can enhance the resistance of A. membranaceus var. mongholicus, promote increases in the biomass of medicinal components and improve the quality of A. membranaceus var. mongholicus.