Abstract:To determine the distribution of fine roots of Robinia pseudoacaciamixed forests and pure forest in saline-alkali soils of the Yellow River Delta, fine root distributions in Fraxinus velutinaand Robinia pseudoacaciamixed forest, Ailanthus altissimaand Robinia pseudoacaciamixed forest and Robinia pseudoacaciapure forest were sampled with a soil column method. The vertical distributions of fine roots in different forest stands were analyzed for the distributions of fine root biomass density, fine root surface area density, volume density, root length density and other root parameters. The aim of the study was to clarify differences of roots distribution of different forests and their relationship with soil properties and to provide references for the vegetation recovery and tree species selection in saline soils of the Yellow River Delta. The results showed that fine root biomass, surface area, volume and root length of F. velutinaand R. pseudoacaciamixed forest were significantly higher than those of A. altissimaand R. pseudoacaciamixed forest and R. pseudoacaciaplantation. About 95.77% of fine root biomass was distributed in the 0-60 cm soil layer for F. velutinaand R. pseudoacaciamixed forest, 85.37% in the 0-40 cm soil layer for A. altissimaand R. pseudoacaciamixed forest and 66.38% in the 0-40 cm soil layer for R. pseudoacaciapure forest. Although surface aggregation of fine roots was conducive to reducing the harmful effects of saline-alkaline conditions on the root, fine roots of R. pseudoacaciapure forest were more uniformly distributed in the soil. F. velutinaand R. pseudoacaciamixed forest had the highest total fine root biomass (91.56 g in 2 500 cm ²of soil), significantly higher than those of other trees stands. Fine root surface area, length and volume densities of F. velutinaand R. pseudoacaciamixed forest were significantly higher than that of R. pseudoacaciapure forest. Fine root tips of F. velutinaand R. pseudoacaciamixed forest and A. altissimaand R. pseudoacaciamixed forest were respectively 2.34 and 1.23 times that of R. pseudoacaciapure forest. Root forks of F. velutinaand R. pseudoacaciamixed forest and A. altissimaand R. pseudoacaciamixed forest were respectively 6.15 and 1.66 times that of R. pseudoacaciapure forest. There was a significant positive correlation between stand fine root biomass with soil available phosphorus and soil available potassium contents. The correlation between fine root biomass with available nitrogen and organic matter content of F. velutinaand R. pseudoacaciamixed forest was also very significant and positive. The research showed that some trees mixed patterns increased fine root biomass distribution and the ability of roots to absorb soil nutrient. This suggested that proper mixing patterns could increase the adaptability of tree plantations.