Seasonal Variation in Leaf Extract NH3 Emission Potential in Fertilized Winter Oilseed Rape

Seasonal changes in leaf chemistry may help characterize ammonia exchange potential in fertilized crops. We investigated leaf-extract ammonium, pH, and emission potential (Γextract = [NH4+]/[H+]) alongside leaf area index (LAI) in winter oilseed rape (Brassica napus L.) near Kartal, Hungary, in 2026. Twenty-eight pooled extracts were analyzed between February and May. Two applications of liquid urea–ammonium nitrate fertilizer each supplied approximately 70 kg N ha−1. Unsmoothed Γextract reached 43,087 after the February application and a smaller local maximum of 17,364 after the March application. Corresponding five-point moving-average maxima were 24,409 and 8533. LAI near the applications increased from 1.10 to 2.14. Semi-quantitative NH3 flux estimates combined measured concentration gradients with a mean diurnal turbulent diffusion coefficient profile obtained at the same site in 2025. Net emission increased following both applications, with estimated cumulative net emission of 13.6 kg N ha−1 between 18 February and 29 June. These estimates represent combined soil–vegetation exchange. Extract apoplastic purity was not independently validated, and the absence of an unfertilized control and replicated treatments precludes causal attribution. The observations motivate testing fertilizer timing relative to canopy development but do not demonstrate management efficacy.