## 4 Discussion
In addition, due to the strong adsorption performance of biochar, it also increases the complexity of soil enzymes. The adsorption of biochar on reaction substrates facilitates enzyme reactions and promotes the soil's enzyme activity, as well as increasing the soil's organic matter content. However, when the amount of added biochar is too high (5%), the organic matter content in the soil significantly decreases (Figure 4A). This may be due to the excessive toxic substances (heavy metals, polycyclic aromatic hydrocarbons, etc.) that spread into the soil from the biochar that inhibits the growth of soil microbes (Figure 2), which reduce the soil's enzyme quantity and the release of the soil's organic matter. These toxic substances also have an inhibitory effect on the soil's enzyme activity. In addition, [PERSON] et al. [39] reported that the adsorption of soil enzymes on biochar can reduce the availability of binding sites, which inhibit enzymatic reactions and the soil's organic matter release. Soil microbes are an important factor affecting soil fertility, and biochar could affect the growth activity of some phosphorus-/potassium-releasing bacteria [14]. In the present study, we found that significant increases of available phosphorus/potassium content in soil with both biochar and microbes when compared with the levels found in soil with either biochar or microbes alone. Presumably, the interaction of biochar with microbes could promote microbial phosphorus-/potassium-solubilising activity, which caused an increase in the phosphorus/potassium content in the soil. The proportion of the _Ochroductrum_ sp. ACCC10085 strain in the soil with the addition of 0.5-2% biochar was relatively high, which could promote soil phosphorus dissolution and increase the available phosphorus content in the soil. However, the proportion of the _Ochroductrum_ sp. ACCC10085 strain in soil with the addition of 5% biochar was decreased, which led to a decrease in phosphorus solubility as well as the available phosphorus content in the soil. This strain content was closely related to the available phosphorus content in the soil (Table 1). The proportion of the _B. mucilaginousus_ AS1.153 strain in soil with the addition of 5% biochar was relatively high, which could promote the dissolution of soil potassium and increase the content of available potassium in the soil. The bacterial content was closely related to the available potassium content in the soil (Table 1). Biochar affects the activity of extracellular enzymes in soil, which are responsible for the degradation of organic carbon and the activity of other important phosphorus-solubilising enzymes or potassium lyase. Soil enzyme activity is influenced by factors such as the pH, cation exchange capacity, water capacity, and pore structure. [PERSON] et al. [40] found that high and low temperature biochar can cause different changes in enzyme activity. Research has shown that the impact of biochar on the soil's enzyme activity varies depending on the type of biochar and soil enzyme type. Biochar contains phosphorus, potassium, magnesium, and other nutrients, which can promote soil microbial activity and thereby improve soil fertility. It was concluded that the interaction of biochar with soil microbes could enhance soil fertility, including organic matter and the phosphorus and potassium content that was able to promote the plant growth of Chinese cabbage. Phosphorus accumulation in Chinese cabbage was significantly higher in the combined treatments of biochar and soil microbes than in the control (Figure 6A). Enhanced phosphorus accumulation by plants as a result of biochar application could be attributed to the high phosphorus content of the corncob biochar and thus higher phosphorus content and availability in biochar-amended soils [41]. Improved soil pH and cation exchange capacity, leading to better nutrient absorption and reduced phosphorus fixation, could also be the case [20].