Objective To investigate the effect of biochar addition at different phosphorus (P) levels on P transformation and crop P uptake in red soil, as well as the underlying processes and mechanisms, and provide a scientific basis for the application of biochar in reducing P inputs and increasing efficiency, as well as promoting soybean P uptake..

Method An acidic red soil was used as the test soil to explore the effects of rice straw biochar 0, 4%(w) under four phosphate fertilizer levels (0, 30, 60 and 90 kg·hm⁻²) on soil components, phosphatase activity, microbial biomass P and soybean P uptake and their correlations.

Result The addition of biochar promoted soybean P uptake and P use efficiency, with a more significant effect at 30 kg·hm⁻² P treatment. Biochar application at different P levels increased soil inorganic P content, especially the NH₄F-Pi、 HCl- Pi. At without phosphate fertilizer treatment, adding biochar reduced NaOH-extractable organic P by 48.5%. Furthermore, adding biochar at without and 30 kg·hm⁻² P treatment increased soil alkaline phosphatase activity by 71.7% and 46.0%, and soil microbial biomass P by 458.3% and 84.0%, respectively.

Conclusion Biochar addition under low P conditions significantly promotes plant P uptake and P use efficiency, while also enhancing the biological transformation of soil labile organic P.