Fine root distribution in mixed Robinia pseudoacacia plantations in saline soils of the Yellow River Delta

To determine the distribution of fine roots of Robinia pseudoacacia mixed forests and pure forest in saline-alkali soils of the Yellow River Delta, fine root distributions in Fraxinus velutina and Robinia pseudoacacia mixed forest, Ailanthus altissima and Robinia pseudoacacia mixed forest and Robinia pseudoacacia pure 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. velutina and R. pseudoacacia mixed forest were significantly higher than those of A. altissima and R. pseudoacacia mixed forest and R. pseudoacacia plantation. About 95.77% of fine root biomass was distributed in the 0-60 cm soil layer for F. velutina and R. pseudoacacia mixed forest, 85.37% in the 0-40 cm soil layer for A. altissima and R. pseudoacacia mixed forest and 66.38% in the 0-40 cm soil layer for R. pseudoacacia pure 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. pseudoacacia pure forest were more uniformly distributed in the soil. F. velutina and R. pseudoacacia mixed forest had the highest total fine root biomass (91.56 g in 2 500 cm² of soil), significantly higher than those of other trees stands. Fine root surface area, length and volume densities of F. velutina and R. pseudoacacia mixed forest were significantly higher than that of R. pseudoacacia pure forest. Fine root tips of F. velutina and R. pseudoacacia mixed forest and A. altissima and R. pseudoacacia mixed forest were respectively 2.34 and 1.23 times that of R. pseudoacacia pure forest. Root forks of F. velutina and R. pseudoacacia mixed forest and A. altissima and R. pseudoacacia mixed forest were respectively 6.15 and 1.66 times that of R. pseudoacacia pure 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. velutina and R. pseudoacacia mixed 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.