## 4 Discussion
Excessive biochar adsorbs nutrients and minerals from the soil, which limits the nutrition of soil microbes and affects their growth [31]. On the other hand, the potential toxicity caused by the emission of volatile pyrolysis products and water-soluble extracts play a leading role in affecting the biomass and composition of soil microbial communities. [PERSON] et al. [32] reported that biochar released high concentrations of phenolic compounds that decreased the number of _Bacillus mucilaginous_s in soil. [PERSON] et al. [33] reported that the biochar components extracted by water in pyrolysis sugarcane and corn cob inhibited the growth of _Leocobacter_ sp. and _Bacillus argabhattai_. Therefore, adding biochar to soil requires consideration of its amount. An appropriate amount of biochar can promote microbial growth, while an excessive amount of biochar can have an inhibitory effect on soil microbes. The optimal amount of biochar added in this study is 2% (Figure 2). Similar results were found by others authors. [PERSON] et al. [34] reported that the application of 1% and 2% (_v/w_) corn residue biochar in calcareous soil increased soil microbial biomass by 20% to 124% compared to that of the control. Adding biochar to soil can alter the living environment of microbes, leading to an increase in the abundance of certain dominant bacteria or inhibition of the growth of certain pathogens. [PERSON] et al. [35] reported that biochar significantly altered the abundance of rhizosphere microbes, with an increase of 100 times in the abundance of phosphorus solubilisation-related aeromonads, sulphate desulfurization-related arthrobacter, and copper-loving bacteria. [PERSON] et al. [36] showed that the relative abundance of fungi and bacteria, as well as the content of PLGA, decreased with the increase of corn biochar application. This can be explained by the alkalinity of biochar, which often inhibits soil microbial activity in the short term, thereby reducing community composition. In this study, _Ochroductrum_ sp. ACCC10085 and _B. mucilaginous_s AS1.153 strains were added to the soil, which became dominant strains in the soil. When adding 0.5-2% biochar, the proportion of the _Ochroductrum_ sp. ACCC10085 strain in the soil was relatively high (Figure 3), and the proportion of the _B. mucilaginous_s AS1.153 strain in the soil was relatively high when adding 5% biochar (Figure 3). Biochar was reported to increase crop productivity accompanied by an improvement in soil fertility [37]. Numerous studies have shown that biochar has a positive impact on soil quality. Applying biochar can increase the soil's organic matter content, thereby improving soil fertility [38]. According to the raw materials and pyrolysis characteristics used in the production of biochar, it can contain some nutrients, but due to its highly porous structure, it can improve the retention of nutrients in the soil. In addition, biochar can reduce soil acidity and increase soil conductivity and cation exchange capacity, thereby improving nutrient availability [6]. The soil's organic matter is an important indicator for soil fertility that mainly comes from organic fertiliser and the residual root and root exudates of the crops in soil [20]. The changes in the soil were examined in different soil treatments to make clear whether the interaction of biochar with microbes can enhance soil fertility. In the present study, we observed that the accumulation of organic matter in soil with both biochar and microbes was higher than that in soil with biochar alone and that presumably the interaction of biochar with microbes could enhance the transformation of the soil's organic matter. This could be because some components in biochar, including minerals, volatile organic compounds, and free radicals, can potentially influence microbial activity, reshape the soil's microbial community, and change the soil's enzyme activity that catalyses various key biogeochemical processes, including the soil's organic matter content and elemental (e.g., N, P, and K) cycles [20].