Straw returning and nitrogen fertilization are important pathways for nitrogen input into soil. This study investigated the effects of straw returning and nitrogen application rates on soybean yield, soil nitrogen components, enzyme activities, and soil nitrogen supply capacity, to provide theoretical basis for soil nitrogen management in soybean cultivation. The experiment was conducted at Daxijiang Farm of Jiusan Branch, Beidahuang Group in 2022 and 2023. In the straw-removed area, a conventional fertilization treatment (N49.2 kg/hm², S0N2) was setup. In the straw-return area, three treatments were designed: no nitrogen application (S1N0), 30% reduced conventional nitrogen (N34.4 kg/hm², S1N1), and conventional nitrogen application (N49.2 kg/hm², S1N2). The results showed that compared with S0N2, S1N2 significantly increased soybean yield by 19.15%-24.51%, with notable enhancements in topsoil nitrogen components (0-20 cm, the same below) and enzyme activities. Under straw return conditions, increased nitrogen application significantly promoted total nitrogen (TN), NO₃^--N, and NH₄⁺-N contents, while exhibiting a "low-promotion and high-inhibition" effect on the contents of acidolytic- total nitrogen (TAN), ammonia nitrogen (ANN), amino sugar nitrogen (ASN), and amino acid nitrogen (AAN), as well as sucrase (SUC), urease (UR), and protease (PR) activities. Correlation analysis revealed highly significant positive correlations between soil UR, SUC, and PR activities with TAN, ANN, and AAN contents, as well as strongest correlations between enzyme activities and organic nitrogen component contents. In conclusion, both straw returning and nitrogen application under straw return conditions improved soybean yield and topsoil nitrogen supply capacity. The 30% reduced conventional nitrogen application (34.4 N kg/hm²) under straw return demonstrated optimal performance in maintaining high soil nitrogen supply capacity while achieving fertilizer reduction and stable soybean yield.