Background and aims <p>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 ¹⁵N tracing experiment.</p> Methods <p>¹⁵N-labeled maize (<i>Zea mays</i> L.) residues were incorporated into the soil, followed by vetch (<i>Vicia faba</i> L.), barley (<i>Hordeum vulgare</i> L.) cover crops, or bare fallow. A subsequent maize crop was fertilized with 210&#xa0;kg N ha⁻¹ as ammonium nitrate (AN) or ¹⁵N-labeled AN (¹⁵NH₄NO₃, ¹⁵AN//NH₄¹⁵NO₃, A¹⁵N) in the microplots receiving ¹⁵N-enriched or unlabeled maize residues, respectively.</p> Results <p>Cumulative N₂O emissions were 47% higher under vetch than under barley. Average N₂O emission factors were 0.05% for ¹⁵N-labeled residues and 0.23% for fertilizer. Most N₂O originated from ¹⁵AN (42%–61%), followed by endogenous sources (i.e., mineralization of organic N from soil or crop residues, 19%–42%) and A¹⁵N (9%–18%). Maize recovered 7.7% of residue N, with no differences between cover cropping treatments. Plant uptake from A¹⁵N was 2.3 times higher than from ¹⁵AN. 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% (<i>P</i> &lt; 0.05) compared with fallow.</p> Conclusion <p>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.</p>