Shift of root nitrogen-acquisition strategy with tree age is mediated by root functional traits along the collaboration gradient of the root economics space.

This article investigates how root nitrogen (N) acquisition strategies vary with tree age in three coexisting ectomycorrhizal coniferous species—Pinus koraiensis (Korean pine), Picea koraiensis (Korean spruce), and Abies nephrolepis (smelly fir)—within a natural broadleaved-Korean pine forest in northeastern China. Using a field isotopic hydroponic method, the study found that total root N and ammonium (NH4+) uptake rates declined with increasing tree age across all species, while preferences for N forms shifted, with most trees favoring NH4+ except middle-aged Korean spruce and mature smelly fir, which preferred glycine; nitrate (NO3−) uptake was consistently lowest. Root morphological and mycorrhizal colonization traits aligned along a 'collaboration' gradient from a 'do-it-yourself' strategy (high specific root length and branching ratio) in young trees to an 'outsourcing' strategy (larger root diameter and higher mycorrhizal colonization) in mature trees, indicating an ontogenetic shift in N acquisition from direct root uptake to greater reliance on mycorrhizal fungi. These findings enhance understanding of belowground nutrient competition, species coexistence, and nitrogen cycling dynamics in temperate forest ecosystems.