## 1 Introduction  
Biochar is a solid carbonaceous residue produced by biomass pyrolysis in an oxygen-free or low-oxygen environment and includes charcoal, rice husk charcoal, and straw charcoal. It has a large specific surface area and porosity and strong adsorption and antioxidant abilities [1]. Different raw materials and preparation conditions can lead to changes in the physical and chemical properties of biochar, such as texture, pH, ash content, and nutrient content, resulting in different environmental effects [2]. Numerous studies have shown that biochar has a positive impact on soil quality [3]. The application of biochar can increase the soil's organic matter content, thereby improving soil fertility [4]. According to the raw materials and pyrolysis characteristics used in the production of biochar, it contains nitrogen, phosphorus, potassium, and other nutrients, which are further released into the soil [5]. In addition, biochar can increase soil alkalinity and increase soil cation exchange capacity and conductivity, thereby improving nutrient availability. Biochar significantly increased the bulk density, texture, pore structure, pore shape, aggregate stability, surface area, and water availability of the soil [6]. Thus the soil improvement with biochar has been used to increase agricultural production, whereas most researches on biochar were focused on soil amendment for promoting crop growth [7, 8].  
Soil microbe is an important participant in soil biochemical process that includes oxidation, nitrification, ammonisation, nitrogen fixation, and sulphuration [9]. It promotes the decomposition of the soil's organic matter and nutrient transformation that has a direct impact on soil fertility [10]. Biochar is a new soil improvement material that can change the number and structure of microbial populations in soil and enhance its function. The specific effects of biochar on microbes can be divided into several categories [11]: (1) biochar with high surface area and porous structure serves as a shelter for soil microbes; (2) biochar provides nutrients such as carbon and nitrogen sources for soil microbes; and (3) biochar improves soil properties and promotes microbial growth. Biochar was able to regulate the microbial community structure and activity through a differential impact on soil microbes in the process of biochar amending soil. Such alteration is beneficial to the construction of the soil environment being conducive to plant growth and increasing soil fertility for crop production [12]. However, the impact of the interaction between biochar and soil microbes on plant growth is rarely reported. Soybean growth could be improved in the biochar-amended soil, and the microbial diversity in the rhizospheric soil was significantly increased, most of which have the characteristics of promoting plant growth and resisting against phytopathogen [13]. [PERSON] et al. [14] reported that corncob biochar could improve potassium-/phosphate-solubilising activities by promoting the bacterial growth of _Ochroductrum_ sp. and _Bacillus mucilaginousus._  
According to the available information, there is no investigation on the promotion of plant growth by the interaction of biochar with microbes to enhance soil fertility. Therefore, the aim of this study was to investigate the effect of the interaction of biochar with phosphorus-/potassium-solubilising bacteria on soil fertility. The other aim was to examine the influence of the interaction between biochar and soil microbes on cabbage growth and quality. The third and the final aim was to explain the impact of interaction between biochar and soil microbes on cabbage growth by increasing soil fertility. This will be beneficial in helping to understand the mechanism of biochar increasing crop yield.