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 input 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 4%(w) rice straw biochar under four phosphate fertilizer levels (0, 30, 60 and 90 kg·hm⁻²) on contents of soil components, phosphatase activity, microbial biomass P content and soybean P uptake and their correlations.

Result P level, biochar and their interaction had significantl effects on soybean P absorption, soil P composition and turnover. The biochar addition 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. Without phosphate fertilizer, adding biochar reduced NaOH-extractable organic P by 48.5%. Furthermore, adding biochar with 0 and 30 kg·hm⁻² P treatment increased soil alkaline phosphatase activity by 71.7% and 46.0% respectively, and soil microbial biomass P content 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.