Objective  The objective was to determine the root nitrogen uptake and its relationship with root morphological and chemical traits in Korean pine (Pinus koraiensis) at different ages, and to advance our understanding in the linkage between root resource acquisition strategy and tree ontogeny.

  Method  We sampled young ((14 ± 1) years), middle-aged ((48 ± 3) years) and mature ((217 ± 4) years) individuals of Korean pine in a mixed broadleaved Korean pine forest at Liangshui National Nature Reserve, Heilongjiang Province of northeastern China. Root nitrogen uptake was measured in situ with a short-term ¹⁵N labeling hydroponic experiment, and root morphological and chemical traits were also determined concurrently.

  Result  Root ammonium, glycine and total nitrogen uptake rate of Korean pine decreased gradually with the increase of age, while no significant change in nitrate uptake rate was found. Across all ages, the contributions of different forms of nitrogen to total uptake ranked in the order of ammonium (62%−65%) > glycine (25%−32%) > nitrate (4%−12%). The contribution of nitrate to total nitrogen uptake increased with the increase of age, but no clear patterns were shown in ammonium and glycine. The percentages of glycine as molecular absorbed by roots were very similar across young, middle-aged and mature individuals, with the corresponding values of 78%, 81% and 80%, respectively. With tree age increased, root diameter increased significantly, showing negative correlations with uptake rates of ammonium, glycine and total nitrogen (significant correlation only found in glycine), but positive correlation with nitrate. By contrast, specific root length and specific root surface area decreased with increasing age, both of them were positively correlated with uptake rates of ammonium, glycine and total nitrogen (significant correlation only found in glycine), but negatively correlated with nitrate. Root tissue density and chemical traits did not show significant changes among tree ages, and exhibited very weak relationships with nitrogen uptake rates.

  Conclusion  With the increase of tree age, root nitrogen uptake rate and preference of P. koraiensis are significantly changed, which may be related to the alternation of root morphological traits.