The fine root biomass and morphological characteristics of Pinus tabulaeformis plantation with different forest age and density in Daqing mountain of Hohhot City, Inner Mongolia were studied, and the relationship between forest age and density and fine roots and soil nutrients was clarified, so as to provide a theoretical basis for the scientific management of Pinus tabulaeformis plantation in Daqing mountain area.

Twenty-four plots of Pinus tabuliformis plantations were established, comprising eight treatment types with different stand age classes and densities: young stands with medium density, young stands with high density, middle-aged stands with low density, middle-aged stands with medium density, middle-aged stands with high density, mature stands with low density, mature stands with medium density, and mature stands with high density. The root coring method was employed to collect fine root and soil samples from two depth layers (0-20 cm and 20-30 cm), investigating fine root biomass, morphological characteristics, and their relationships with soil nutrients under different stand ages and densities.

The fine root biomass, root length density and root surface area density were concentrated in the 0-20 cm layer. In young forests, with the increase of density, fine roots showed an increasing trend. From middle-aged forest to mature forest, the fine roots showed a trend of increasing first and then decreasing with the increase of density. The total biomass, root length density and root surface area density of fine roots reached the maximum of 411.60 g/m², 830.87 m/m² and 224.05 m²/m³ at medium density in middle-aged forest. Soil organic matter, total nitrogen, total carbon, available nitrogen and available phosphorus content were positively correlated with fine root biomass, root length density and root surface area density, but negatively correlated with fine root specific root length and specific surface area.

In the scientific management of Pinus tabulaeformis plantation in Daqing mountain, combined with the synergistic effect of forest age and density, high-density planting mode should be adopted in the young forest stage, and medium-density planting mode should be adopted in the middle-aged forest and mature forest stage, which is more conducive to promoting the growth and development of trees.