Aiming to investigate the effects of combined straw and biochar application on soil nitrogen pools, crop yield, and nitrogen utilization in the black soil region of Northeast China, a split-plot experimental design was employed, incorporating three field return methods—control (CK, no return), full straw return (SF), and combined biochar and straw return (BS)—along with three nitrogen application rates: 75kg/hm2 (N1), 60kg/hm2 (N2), and 45kg/hm2 (N3). The results showed that the mixed application of straw and biochar alleviated the unfavorable impression of reduced nitrogen application on soil nitrogen content and soybean plant growth, and the effects were more pronounced at N2 nitrogen application. Compared with CK and SF treatments, BS treatment promoted the increase of soil ammonium nitrogen and nitrate nitrogen contents by 7.58%~78.08% and 19.02%~95.56%, respectively, which significantly enhanced the net photosynthetic rate and nitrogen utilization efficiency of soybean, and led to the increase of soybean yield by 38.62%~60.97%. Comprehensive evaluation using the entropy-weighted TOPSIS model identified the BSN2 treatment as the most effective, achieving an average two-year yield of 3058.48kg/hm2 with a nitrogen application rate of 60kg/hm2. This treatment also recorded high nitrogen use efficiency (0.99), agronomic efficiency (9.34kg/kg), nitrogen recovery rate (0.98), and nitrogen response index (2.18). These findings can provide a scientific basis for optimizing nitrogen fertilizer management and enhancing straw utilization in Northeast China’s black soil region.