The study investigated the impacts of biochar (BC) and biological nitrification inhibitors (BNIs), such as methyl 3-(4-hydroxyphenyl) propionate (MHPP), on soil nitrogen transformation in calcareous purple soil, which included both aerobic ¹⁵N labeling experiment and anaerobic acetylene (C₂H₂) inhibition experiment to examine the effects of different BC applied rates (10 g/kg and 50 g/kg) and/or MHPP on gross nitrogen transformation rates, including mineralization rate, nitrification rate, denitrification rate, and NH₄⁺ immobilization rate. The results showed that, compared to the control group, mineralization rates increased by 26.63%-60.06% under 1%BC (10 g/kg), 5%BC (50 g/kg), and 5%BC+MHPP treatments. Biochar applied alone (1%BC and 5%BC) had no inhibitory effect on nitrification rate, while the combined treatments of 1%BC+MHPP and 5%BC+MHPP significantly reduced nitrification rates by 50.34% and 61.56%, respectively (P<0.05). NH₄⁺ immobilization rates increased significantly 95.88%-217.53%(P<0.05) under all treatments with BC, with the highest increase observed under 5%BC treatment. Additionally, biochar showed the potential to stimulate the denitrification process, The lowest ratios of nitrification rate to mineralization rate and nitrification rate to NH₄⁺ immobilization rate were observed under 5%BC+MHPP treatment. These findings suggest that the combined application of 5%BC and MHPP may be an effective strategy to reduce nitrogen loss in calcareous purple soil and enhance crop nitrogen utilization efficiency, but further field validation is warranted to confirm the effectiveness of this approach.