Adsorption And Biochemical Degradation Of Ginsenoside Rb₁ By KHB₄-modified Biochar

Saponins are secondary metabolites produced by many medicinal plants during the growth process,which enter the external environment through root secretion,leaching,volatilization,and decay of residual plants,resulting in a series of negative effects such as slowing seed germination,reducing crop yield,interfering with the growth of neighboring plants,and even reducing soil fertility,disrupting the balance of soil microbial communities,and causing tillage disorders.Continuous cropping obstacles caused by saponins have become one of the important factors restricting the sustainable development of the medicinal plant industry and land resource management.The current methods for remedying soils with continuous cropping obstacles include biological control,crop rotation and intercropping,and soil disinfection.However,all the methods can’t fundamentally regulate the saponins secreted by plants and accumulated in the soil,and the continuous cropping obstacle would still recur.Biochar,as a low-cost,high-performance soil remediation material,has great potential for remediating continuous soil crop obstacles,and its properties can be targeted and optimized through a series of modifications to solve the continuous cropping problem of saponin source soil in a more targeted way.Therefore,we selected the reducing agent KHB₄ to modify the biochar,which was carbonized at different temperatures（350°C and 700°C）and subsequently washed the prepared biochar.The physicochemical properties of biochar were analyzed,and the adsorption and degradation effects of KHB₄-modified biochar on the ginsenoside Rb₁,a typical allergenic substance,were investigated.Secondly,the effects of KHB₄-modified biochar on the structure and diversity of microbial communities in different soil types were investigated.Finally,the degradation of ginsenoside Rb₁ by different biochar-mediated microorganisms in different soil types was investigated.The results can provide a theoretical basis and a new idea for the remediation and relief of soil continuous cropping obstacles induced by saponins.The main conclusions are as follows:（1）The adsorption effect of KHB₄-modified biochar on ginsenoside Rb₁ is much better than that of unmodified biochar（p<0.01）,with the highest adsorption amount reaching about 50 mg/g.Modified with KHB₄ increases the specific surface area and the pore structure richness of biochar,resulting in more aromatic adsorption sites,and the pore-filling mechanism is enhanced;Modified with KHB₄ increased the number and type of oxygen-containing functional groups on the surface of biochar,and these oxygen-containing functional groups could form electron-donor-acceptor complexes with saponins to promote saponin adsorption.Adsorption kinetic data indicate that the adsorption of ginsenoside Rb₁ by KHB₄-modified biochar is a chemisorption,ion exchange-driven process controlled by both reaction rate and diffusion factors.（2）The chemical degradation of ginsenoside Rb₁ by KHB₄-modified biochar was significantly enhanced compared with unmodified biochar.This may be due to the increase of oxygen-containing functional groups on the surface of biochar due to KHB₄modification,which enhanced the redox ability and promoted the electron transfer between biochar and ginsenoside Rb₁.（3）The application of KHB₄-modified biochar helped to increase microbial community diversity,enhance the abundance of beneficial microorganisms in the four soils tested.The abundant pore structure on the surface of KHB₄-modified biochar is not only suitable for beneficial microbial colonization but also can store the anions and cations,water,and soluble substances required for microbial metabolism adsorbed by the biochar itself using the surface oxygen-containing functional groups.Moreover,KHB₄-modified biochar itself can provide a certain amount of carbon and nutrients,which can provide conditions to promote microbial reproduction,enhance microbial activity and improve microbial diversity.The application of KHB₄-modified biochar can help create a reducing environment in the soil,and it was pointed out that the reducing environment was conducive to the increase of the relative abundance of beneficial genera such as Bacillus,Paenibacillus and Klebsiella in the soil.Meanwhile,the growth of major pathogenic bacteria of Panax notoginseng such as Fusarium and Alternaria would be inhibited in this environment,which was also consistent with the results of this experiment.（4）The degradation of ginsenoside Rb₁ by microorganisms in the four soils under the mediation of KHB₄-modified biochar was better than that of the applied unmodified biochar and unapplied biochar,with the highest degradation amount reaching about 95mg/g.The relative abundance of beneficial genera such as Bacillus,Paenibacillus,and Talaromyces increased in each soil under the regulation of KHB₄-modified biochar,and the content of these genera was negatively correlated with the content of saponin accumulated in the soil,presumably these genera were involved in the biodegradation of saponin.