Objective  The root decomposition and carbon and nitrogen nutrient release characteristics of different tree species in cold temperate zone were discussed in order to provide scientific reference for understanding the root decomposition and carbon and nitrogen nutrient cycling of tree species in cold temperate zone.

  Method  Mesh bag decomposition method was adopted from August 2020 to August 2021. The decomposition rates and the dynamics of carbon and nitrogen elements of fine roots (≤ 2 mm), middle roots (2−5 mm) and coarse roots (5−10 mm) of Betula platyphylla, Populus davidiana, Larix gmelinii and Pinus sylvestris var. mongolica were studied for one consecutive year in the Daxing’an Mountains region of northeastern China.

  Result  (1) After 365 d of decomposition, the annual root decomposition rates of Betula platyphylla, Populus davidiana, Larix gmelinii and Pinus sylvestris var. mongolica varied in 29.75%−39.92%, 33.90%−39.45%, 22.08%−28.80% and 22.62%−33.75%, respectively. The root decomposition rate of different diameter classes was fine root > medium root > coarse root, Betula platyphylla and Populus davidiana were higher than those of Larix gmelinii and Pinus sylvestris var. mongolica. The decomposition rates were higher at the initial stage of decomposition (0−61 d) and lower at 243−365 d. (2) Olson exponential decay model was used to calculate the annual root decomposition coefficient (k) of each tree species. It was found that: the decomposition coefficient of fine root of Betula platyphylla was the highest, which was 0.508 5. The decomposition coefficient of fine root of Betula platyphylla and Populus davidiana was significantly higher than that of Larix gmelinii and Pinus sylvestris var. mongolica in the same diameter class, and the decomposition coefficient of middle root and coarse root of Pinus sylvestris var. mongolica was only about 50% of that of Populus davidiana. (3) At different decomposition stages, root carbon and nitrogen of all diameter sizes of the four tree species showed different degrees of release or enrichment. The release rate of carbon and nitrogen was the highest at the initial stage of decomposition (0−61 d), the release rate of carbon was lower at 243−273 d decomposition stage, and the enrichment of nitrogen was higher at 304−365 d decomposition stage. After one year of decomposition, the release rates of carbon and nitrogen were in the range of 18.68%−34.21% and 14.90%−28.92%, respectively. The total carbon and nitrogen release rates of Betula platyphylla and Populus davidiana were higher than those of Larix gmelinii and Pinus sylvestris var. mongolica. The release rate of carbon from fine roots was the highest, while the release rate of nitrogen from medium and coarse roots was relatively low.

  Conclusion  The decomposition rate and nutrient release rate of carbon and nitrogen of roots of different tree species and diameters differ greatly in different stages. The decomposition rates and carbon and nitrogen nutrient release rates of Betula platyphylla and Populus davidiana are higher than those of Larix gmelinii and Pinus sylvestris var. mongolica.