Abstract: The nutrient absorption of N,P and K in root systems of Populus×xiaozhuanica and Robinia pseudoacacia is analysed by a dynamic methodology.The results show that:1) NH₄⁺ affinity for Populus×xiaozhuanica is weaker than that for Robinia pseudoacacia;the rate of NH₄⁺ flowing into Populus×xiaozhuanica is slower than that flowing into Robinia pseudoacacia;the maximum absorption rate of NH₄⁺ by the root system of Populus×xiaozhuanica is much higher than that of Robinia pseudoacacia;compared with Robinia pseudoacacia,Populus×xiaozhuanica has a stronger absorbency for and resistance to a deficiency in NH₄⁺.2) the rate of H₂PO₄^-flowing into Robinia pseudoacacia is higher than that of Populus×xiaozhuanica;Robinia pseudoacacia has a stronger H₂PO₄^-deficiency resistance than Populus×xiaozhuanica.3) although differences exist between Populus×xiaozhuanica and Robinia pseudoacacia in the dynamic characteristics of the ability to absorb K⁺,competition does not occur when the two species are planted together.Populus×xiaozhuanica and Robinia pseudoacacia share a complementary and harmonious relation in the absorption of N and P nutrient elements.