Introduction: Variation in plant nitrogen uptake rate and substrate preference is complicated due to the combined influence of abiotic and biotic factors. For the same species of tree across different ages, the interactions between root structural traits, nitrogen uptake rate, and soil environment have not been fully characterized, a situation that constrains our understanding of underground resource strategies employed by trees at different ages.Methods: In the present study, we examined the nitrogen uptake rate, mycorrhiza, morphology, architecture, and chemistry of the roots of Larix principis-rupprechtii in a chronosequence (aged 18, 27, 37, 46, and 57 years) in the Saihanba Mechanical Forest Farm in Northern China.Results: L. principis-rupprechtii preferred to absorb ammonium, followed in order by glycine and nitrate. The ammonium uptake rate of L. principis-rupprechtii significantly decreased (aged 18–37 years) and then increased (aged 46–57 years) with tree age. The glycine, nitrate, and total nitrogen uptake rates decreased with tree age. The root resource acquisition strategy appeared to shift from an acquisitive strategy to a conservative strategy associated with increasing tree age.Discussion: Along the root-mycorrhizal collaboration gradient, the younger L. principis-rupprechtii relied more on their own root morphology and physiology to acquire resources, adopting a “do it yourself” strategy comprising increasing the specific root length, the specific root area, and the N uptake rate (nitrate and glycine). Conversely, older trees depended more on mycorrhizal partners to acquire nitrogen resources, an “outsourcing” strategy. The results contribute to our understanding of underground resource-use strategies of plants and the nitrogen cycle in forest ecosystems according to stand age.

The soil microbial community composition and Alpha diversity serve as key indicators of soil quality changes driven by forest development. We explored the variations in soil properties, root traits, microbial communities, and their interrelationships across forest age and soil depth in Populus tomentosa , Platycladus orientalis , and Styphnolobium japonicum plantations. The results showed that the Chao, Shannon, and Pielou_e indices of the soil microbial community increased with forest age. Soil properties and root traits had a stronger influence on the composition of soil bacteria (41.4%) compared to fungi (28.8%). In comparison to root traits (7%–10%), soil properties had a more significant influence (23%–26%) on microbial composition. Soil clay, water content, and conductivity showed positive effects on bacterial diversity and composition, while fungi were mainly affected by soil total phosphorus and soil pH. The influence of root traits on bacterial diversity declined with forest age, whereas the effect of soil properties increased. Fungal diversity was jointly shaped by soil properties and root traits in 13–19a plantations, but mainly by soil properties in 9–12a and 16–36a plantations. With increasing soil depth, the impact of roots on bacterial diversity grew while on fungal diversity diminished. The results highlight the need to account for forest age and soil depth when revealing the association among soil microbial diversity, environmental variation, and root traits.