Context: Diversified crop rotation is crucial for improving agricultural nitrogen utilization efficiency (NUE) and mitigating environmental pollution. However, the mechanisms by which preceding crops regulate N supply and utilization through residual N effects remain inadequately understood. Research questions: Within long-term rotations, how do different preceding crops influence the fate of fertilizer N (including its residual effect), regulate soil N supply, and consequently determine the N uptake and utilization efficiency of subsequent crops? We hypothesized that a shift in preceding crop species would improve soil residual N retention and availability, which in turn would promote residual N utilization by the following crop, leading to enhanced NUE and reduced N loss at the system level. Method: Using continuous 15N isotopic labeling and micro-plot techniques in typical Loess Plateau cropping systems (oilseed flax, potato, and wheat), we systematically quantified the fate of current and residual fertilizer N. Results: Our results showed substantial increases in oilseed flax grain yield (47.7-68.0 %) and N accumulation (45.1-61.7 %) under crop rotations compared to continuous cropping. Although soil-derived N constituted the majority of crops uptake (61.1-78.3 %), fertilizer N contributed substantially: 17.6-23.0 % from current-season and 7.2-16.7 % from residual fertilizer N. Crop rotations enhanced plant N accumulation from current and residual fertilizer N by 51.2-89.2 % and 48.3-255.0 %, respectively, with potato as the preceding crop showing the strongest residual N utilization. Potato served as the preceding crop (FWPF, WFPF) improved seasonal N recovery and residual N retention, thereby supplying more N to subsequent flax and reducing N loss. The FWPF system achieved the highest residual N utilization efficiency. In contrast, the WFPF and WPWF systems significantly enhanced the current-season fertilizer N utilization efficiency of oilseed flax than other treatments, while also exhibiting lower N loss rates compared to continuous cropping. Conclusion: Crop rotations enhance NUE and mitigate N loss by enhancing N uptake, reprogramming N utilization patterns, and improving synchronization of N supply between preceding and subsequent crops. Implication: These findings establish a theoretical and practical basis for designing sustainable, high-NUE rotations in Loess Plateau dryland agriculture.