Season exerts stronger control than tree species on nitrogen cycling in the rhizosphere of young alder and oak forests

Abstract Aims Understanding nitrogen (N) dynamics in young forest plantations is critical for predicting long-term soil fertility and greenhouse gas emissions. Methods We investigated seasonal patterns of N transformation, extracellular enzyme activities, and soil respiration in rhizosphere soils of alder ( Alnus glutinosa , N-fixing) and oak ( Quercus robur , non-N-fixing) in a ~ 5-year-old mixed temperate plantation. Results Seasonal dynamics strongly regulated N cycling, whereas species-specific effects were less pronounced. Gross N mineralization peaked in spring, closely associated with microbial biomass and N-acquiring enzyme activity, but declined through summer and autumn. Gross nitrification showed a contrast pattern, resulting in a clear decoupling between mineralization and nitrification, likely reflecting stronger microbial immobilization and rhizosphere-mediated regulation in spring, supported by higher microbial biomass, which both weakened toward autumn. Net and gross N transformation differed markedly, with net mineralization highest in autumn, while net nitrification peaked in spring, and both were lowest in summer. Potential N₂O emissions also varied seasonally and between species, being highest in summer for alder and in spring for oak, and lowest in autumn for both. These findings highlight species-specific regulation in N cycling. Species comparisons revealed that alder soils, despite their N-fixing capacity, exhibited tighter microbial–plant coupling with higher N immobilization and lower nitrification, except in spring, while oak soils accumulated more NH₄⁺ and supported greater nitrification, reflecting contrasting rhizosphere strategies. Conclusions Collectively, our findings demonstrate that seasonal dynamics, more than tree species identity, dominate rhizosphere N cycling in young plantations, with implications for nutrient retention, forest productivity, and climate feedback.