Contribution to N2O emissions and N fate of crop residues and synthetic N fertilizer in a cover crop–maize rotation: A 15N-tracing study

Abstract 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 (N 2 O) emissions, particularly in semiarid croplands, remain underexplored. This study aimed to evaluate the N dynamics and N 2 O sources in an irrigated cover crop-maize rotation in central Spain by conducting a 15 N tracing experiment. Methods 15 N-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 15 N-labeled AN ( 15 NH 4 NO 3 , 15 AN//NH 4 15 NO 3 , A 15 N) in the microplots receiving 15 N-enriched or unlabeled maize residues, respectively. Results Cumulative N 2 O emissions were 47% higher under vetch than under barley. Average N 2 O emission factors were 0.05% for 15 N-labeled residues and 0.23% for fertilizer. Most N 2 O 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 N 2 O emissions in semiarid agro-ecosystems.