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.