Objective  As the main organ for nutrient and water absorption, fine roots are the most active part of the plant root system and particularly sensitive to soil environment. Nitrogen (N) deposition changes the soil environment, and would certainly affect the structure and function of fine roots. Pinus koraiensis (PK) is an important species in broadleaved Korean pine forest. It is important to discuss the effects of different levels of N addition on the stoichiometry, morphological traits and anatomical structure of PK fine roots, understanding the variation and trade-offs of fine root traits under long-term N addition, and predicting the physiological function changes of plant roots under global climate change.

  Method  Three sample plots were established in primary mixed coniferous forests in the Xiaoxing’an Mountains of northeastern China, four treatments were set up in July 2011, which were the control (CK, 0 g/(m²·year)), low nitrogen treatment (LN, 2.5 g/(m²·year)), medium nitrogen treatment (MN, 5.0 g/(m²·year)) and high nitrogen treatment (HN, 7.5 g/(m²·year)). Fine roots of PK were dug out in July 2019, and 1st−5th fine roots under N addition were determined, including fine root stoichiometry, fine root diameter, root length, root surface area, cortex thickness, cortical cell traits, stele diameter, ratio of stele to root diameter and tracheid traits.

  Result  Compared with CK treatment, N addition significantly increased the TN content of PK fine roots, LN and MN treatments significantly reduced C∶N; LN and MN treatments significantly increased the diameter of 1st−5th fine roots and the mean length and surface area of 1st fine roots, but reduced the mean length and surface area of 2nd−3rd fine roots. LN treatment significantly increased the 1st−5th fine root stele diameter, and 1st−3rd fine root cortex thickness and cortical cell traits, and 4th−5th fine root tracheid traits.

  Conclusion  Long-term N addition significantly changes the stoichiometry, morphological traits and anatomical structure of fine roots. The variation of fine root diameter is mainly derived from cortical thickness and stele diameter, the variation of cortical thickness is mainly derived from cortical cell diameter, and the variation of stele diameter is derived from mean tracheid diameter and total tracheid area, and fine roots respond to environmental changes by adjusting their morphology and anatomical structure to optimize their physiological functions.