Abstract: Nitrogen (N) availability and retention in soil-crop systems are important for increasing crop productivity, improving N use efficiency (NUE), and minimizing environmental pollution from N losses. In the black soil region of Northeast China, it is unclear how agricultural management practices affect soil mineral N accumulation and leaching. In this study, an in-situ ¹⁵N-labeled tracer field experiment was performed to quantify the transformation characteristics, migration, and soil N fate when long-term no-till with maize stover mulching was used. The soil profile was investigated under three treatments: conventional ridge tillage (RT), no-till with no maize stover mulching (NT0), and no-till with 100% maize stover mulching (NT100; 7500 kg·hm^-2 maize stover). The accumulated mineral N primarily as nitrate nitrogen (NO₃^--N) in the 300 cm soil profiles were 461.6 kg(N)·hm^-2 (RT), 450.7 kg(N)·hm^-2 (NT0), and 439.7 kg(N)·hm^-2 (NT100) when traditional fertilizer applications were used, suggesting a N leaching risk. In all 0–40 cm soil layers, the percentage of fertilizer-derived NO₃^--N to total NO₃^--N was on average 60.9% (maize seedling stage) and 58.0% (maize tasseling stage), indicating a high N leaching risk in the seasonally applied fertilizer. NT100 decreased the transformation of fertilizer N into mineral N pools by 20.8% in 0–40 cm soil layers but accelerated the conversion into fixed ammonium NO₃^--N and organic N pools by 39.4% and 30.5%, respectively, compared with that by RT. The clay mineral to fertilizer-derived NO₃^--N fixation capacity was the same as the soil microorganism to fertilizer-derived mineral N immobilization capability at a depth of 0–20 cm, but the fixation capacity was higher than the immobilization capability at 20–40 cm. These findings suggest that the immobilization potential of soil microorganism to fertilizer-derived mineral N is dependent on the maize straw mulch quantity and maize straw accessibility to soil microorganisms. No-till with maize stover mulching reduced the soil mineral N accumulation in black soil spring maize fields, increased the fertilizer nitrogen use efficiency and maize yield by 9.7%, decreased the fertilizer N gaseous loss by 27.7%, and delayed fertilizer N leaching to deeper soil.