Nitrogen supply modulates carbon-nitrogen balance and metabolic profiles in contrasting onion varieties

Onion bulb quality is largely defined by non-structural carbohydrates, phenolic antioxidants, and sulfur-containing flavor precursors, yet integrative evidence on how these domains respond to nitrogen imbalance under controlled conditions remains limited. A pot experiment applied four ammonium nitrate levels (N1-N4; 0.5-4.0 g NH₄NO₃ per pot) to two contrasting onion varieties (hybrid Hytech F1 and landrace Birnenförmige). Bulb morphology and quality traits were quantified alongside tissue elemental C:N ratios and a targeted trait-metabolite matrix covering carbohydrates, amino acids (including Allium-specific alk(en)yl cysteine sulfoxides, ACSOs), organic acids and other primary metabolites, as well as flavonoids, driven by higher dry matter and fructan-rich reserve pools in Birnenförmige versus higher soluble sugars and nitrogen-status metabolites in Hytech. Along the nitrogen gradient, bulb growth followed a non-linear response, peaking at intermediate supply and declining under oversupply, with a stronger high-N penalty in Hytech. Nitrogen oversupply increased nitrate and free amino acid pools and lowered elemental bulb C:N ratios, while total phenolic content and quercetin glycoside levels declined. In contrast, relative levels of putatively annotated ACSO-like compounds increased as pyruvate-based pungency decreased, consistent with a compositional decoupling between sulfur-containing compound profiles and the pyruvate-based pungency signal under high N. Together, the results identify variety-specific thresholds and coordinated trade-offs linking nitrogen status to growth and the major quality-related metabolic domains in onion bulbs.