Stand development shifts nitrogen cycling strategies and supply-demand balance in larch plantations.

Nitrogen (N) often constrains forest productivity, yet changes in N-use strategies and ecosystem N balance during stand development remain poorly understood in cold-temperate plantations. In this study, N stocks and key fluxes were quantified across vegetation, litter, and soil in larch (Larix principis-rupprechtii) plantations representing four stand ages (11, 23, 37, and 47 years) in northern China. Nutrient strategy indices were calculated to characterize N acquisition and internal cycling, and piecewise structural equation modeling (pSEM) was used as a descriptive path-analysis framework to summarize multivariate associations. Tree-layer biomass and N stocks increased across the chronosequence: tree-layer biomass peaked at 256.77 t·hm⁻² at 37, while tree-layer N stocks reached their maximum of 0.94 t·hm⁻² at 47 years. Soil dominated ecosystem N storage (> 87%) across all stand ages. Soil available N stocks consistently exceeded annual plant N uptake, with the largest surplus approximately 0.01 t·hm⁻² in the 37-year-old stand, suggesting that N supply was generally sufficient to meet plant demand during the study period. Nutrient-use indices indicated a developmental shift from relatively greater reliance on N acquisition in younger stands toward stronger internal N recycling in older stands, as reflected by increased litter N return and higher N resorption efficiency. Nitrogen use efficiency reached its highest observed value in the 37-year-old stand. In the pSEM framework, N acquisition was more closely associated with plant nutrient status, whereas N cycling was more strongly associated with stand age and litter nutrient status. These findings suggest that stand development plays an important role in regulating N-use strategies in larch plantations.