Background and aims Crop residue retention and cover cropping are key strategies in conservation agriculture, but their effects on nitrogen (N) cycling and nitrous oxide (N2O) emissions, particularly in semiarid croplands, remain underexplored. This study aimed to evaluate the N dynamics and N2O sources in an irrigated cover crop-maize rotation in central Spain by conducting a N-15 tracing experiment. Methods N-15-labeled maize (Zea mays L.) residues were incorporated into the soil, followed by vetch (Vicia faba L.), barley (Hordeum vulgare L.) cover crops, or bare fallow. A subsequent maize crop was fertilized with 210 kg N ha(-1) as ammonium nitrate (AN) or N-15-labeled AN ((NH4NO3)-N-15, (15)AN//(NH4NO3)-N-15, A(15)N) in the microplots receiving N-15-enriched or unlabeled maize residues, respectively. Results Cumulative N2O emissions were 47% higher under vetch than under barley. Average N2O emission factors were 0.05% for N-15-labeled residues and 0.23% for fertilizer. Most N2O originated from (15)AN (42%-61%), followed by endogenous sources (i.e., mineralization of organic N from soil or crop residues, 19%-42%) and A(15)N (9%-18%). Maize recovered 7.7% of residue N, with no differences between cover cropping treatments. Plant uptake from A(15)N was 2.3 times higher than from (15)AN. Across barley, vetch, and fallow, total N recovery averaged 33% in plants and 22% in soil. Barley cover cropping improved fertilizer N recovery in maize by 57% (P < 0.05) compared with fallow. Conclusion These findings highlight the role of cereal cover crops and the regulation of ammonium oxidation-based processes in enhancing N use efficiency and reducing fertilizer-derived N2O emissions in semiarid agro-ecosystems.