The study investigates the impact of biochar (BC) and biological nitrification inhibitors (BNIs), such as methyl 3-(4-hydroxyphenyl) propionate (MHPP), on soil nitrogen transformation in calcareous soil. This research includes both aerobic 15N labeling experiments and anaerobic acetylene (C2H2) inhibition experiments to examine the effects of BC and/or MHPP on gross nitrogen transformation rates, including mineralization (m_gross), nitrification (n_gross), NH4+ immobilization (i_NH4+), and soil denitrification rate (d). The results showed that, compared to the control group, the m_gross rates increased by 1.27-1.60 times in the 1% BC, 5% BC, and 5% BC_MHPP treatment conditions. Biochar applied alone (1% BC and 5% BC) had no inhibitory effect on the n_gross rate. However, the combined treatments of 1% BC_MHPP and 5% BC_MHPP significantly reduced the n_gross rate by 50.34% and 61.56%, respectively (p < 0.05). The i_NH4+ rates in all treatments increased significantly (2.01-3.23 times, p < 0.05), with the highest increase observed in the 5% BC treatment. Additionally, biochar showed the potential to stimulate the denitrification process, leading to increased d rates in all treatments. The lowest N/M (the ratio of nitrification to mineralization) and N/I ratio (the ratio of nitrification to NH4+ immobilization) were observed in the 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 soil and enhance crop nitrogen utilization efficiency. Further field validation is warranted to confirm the effectiveness of this approach.