Effects of Nitrogen Nutrition on Accumulation and Speciation of Sulfur Compounds in Garlic Under Field Conditions

Nitrogen (N) supply can influence the accumulation of sulfur (S)-containing metabolites that contribute to garlic (Allium sativum L.) flavour and quality, yet evidence of these responses under field conditions remains limited. Nine N application rates (0-360 kg ha^-1) were evaluated under field conditions to examine the relationships between N supply, plant N and S status, and bulb S-containing metabolites. Allicin was quantified across three varieties, while alliin, γ-glutamyl-S-allyl-L-cysteine (GSAC), and alliin-to-allicin conversion were analysed in Glenlarge. The bulb alliin and allicin concentrations increased linearly with the N rate and were positively associated with the bulb N and S concentrations. The foliage N concentration was strongly positively correlated with the bulb alliin and allicin concentrations, suggesting that leaf N status may provide a useful predictor of S-containing metabolite accumulation. Sulfur K-edge X-ray absorption near-edge structure spectroscopy showed that the relative proportions of GSAC and alliin remained relatively consistent across the six N rates. This indicates that N supply was associated with the increased accumulation of S-containing compounds, without a substantial shift in their relative S speciation. The alliin and allicin concentrations increased up to the highest N rate tested (360 kg N ha^-1), whereas increases in allicin yield became relatively small above approximately 200 kg N ha^-1. Overall, these findings provide field-based evidence linking N nutrition with the accumulation of bioactive S-containing compounds in garlic and provide insights into how N supply may influence garlic quality under field conditions.