<p>Biochar plays a key role in enhancing crop yield and improving soil quality. However, after biochar is applied to soil, a process known as aging may alter its physical and chemical properties over time. Because natural aging processes take a long time, artificial aging methods have been explored to shorten the aging period. Additionally, the distinction between changes in soil fertility and soil nutrient status, particularly soil phosphorus levels and bacteria related to the phosphorus cycle, remains unclear. In this study, a pot experiment was conducted to assess the effects of fresh, washed and oxidized biochar on phosphorus (P) bioavailability, bacterial community structure, and pakchoi growth in acidic soil. We simulated biochar aging in the laboratory through leaching and oxidation processes. Biochar, prepared from rice straw, was leached using deionized water to produce washed biochar (WB) and oxidized with hydrogen peroxide (H₂O₂) to produce oxidized biochar (OXB). Our results showed changes in surface functional groups (-COOH and -OH) and modifications in pore structure on the biochar surface after aging. WB and OXB treatments increased pakchoi growth and biomass. The maximum quantum efficiency of photosystem II (<i>Fv</i>/<i>Fm</i>), photosynthesis rate, stomatal conductance, and water use efficiency (WUE) significantly increased with OXB treatment. Soil microbial abundance and diversity of bacteria were significantly increased by biochar amendments (particularly OXB). At the same time, the relative abundance of the phyla Proteobacteria and Actinobacteria significantly increased. Furthermore, network analysis revealed that WB and OXB treatments increased the scale and complexity of the microbial co-occurrence network. Overall, the findings of this study provide a basic theory for improving soil fertility and soil nutrient status by biochar application.</p>