Abstract:To compare the pure plantations of Robinia pseudoacacia, Fraxinus, Ulmus pumila, and Ailanthus and mixed plantation with a nitrogen fixation species of Robinia pseudoacacia in the annual variations of soil water and soil salt, a series of experiments were carried out to measure the growth of the stand and the amount of soil water and soil salt for different kinds of plantations. The results showed that the mixed plantation of R. pseudoacacia and A. altissima had the best growth among three mixed plantations and all pure plantations. A major characteristic of mixed plantations is the improvement of soil water and soil salt distribution. Top layer of soil (0-60 cm) maintained more water in mixed plantations compared to their respective pure plantations, while the deeper soil water was low in all mixed plantations. As expected, there were not significant differences in soil water between pure and mixed plantation in August. Mixed plantations decreased soil salt and changed the annual distribution in different soil layers. Soil salt varied with months of the year, soil depth, and tree species of mixed plantation. For all plantations, soil salt migrated from top to deeper soil. Further more, the mixed plantations lowered soil salt compared to the pure plantation. Contrast to dry season, soil salt was low in rainy season in both pure plantations and mixed plantations, and the vertical distribution was same in both of pure and mixed plantations. Soil water and soil salt distribution were closely related to seasoning precipitation, and this explains that precipitation was the main water source for pure and mixed plantation.