Straw return is a pivotal management practice for optimizing agroecosystem functions and mitigating soil degradation, playing a vital role in sustainable agriculture. However, most studies focus on short-term effects, overlooking the time-dependent threshold and the divergent soil-plant responses across different initial fertility levels. This study was conducted in Kailu (low-fertility) and Arun (high-fertility), Inner Mongolia, with 3-, 5-, and 7-year (3a, 5a, and 7a) continuous maize straw return, to evaluated variations in spring maize yield, nitrogen uptake, soil nitrogen content, and soil enzyme activities. The results showed that 7a straw returned significantly increased yield by 17.86% in Kailu and 13.82% in Arun, with the highest sustainable yield index (SYI) in Kailu. Notably, 7a straw return significantly increased after silking dry matter and nitrogen accumulation by 27.27% and 40.32% in Kailu, respectively. After 7a straw return, total nitrogen, alkaline hydrolysable nitrogen, ammonium nitrogen, nitrate nitrogen, and the activities of urease, protease, sucrase, and cellulase were significantly increased in the 20-40 cm layer, thereby expanding the soil N storage. Random forest and structural equation model analyses showed that straw return duration and dry matter accumulation significantly affected yield in Kailu, whereas soil enzyme activity was the key factor in Arun. This study clarified the optimal straw return duration and driving mechanism for different fertility soils, providing a theoretical basis for sustainable, high-yield spring maize production and targeted soil fertility enhancement in Inner Mongolia's semi-arid region.