Abstract: In this paper,the contents of free amino acid and phytohormone IAA in root suface rhizospheric and rhizoplane soils of Populus bejingensis, Robinia pseudoacacia and Sophora japonica are analysed, and the (contents of nitrogen, phosphorus, potassium and chlorophyll in poplar leaves and their net photosynthetic rate (NPR) and stomatal resistance (SR) under different treatments (pure poplar or mixed with leguminous trees) are measured. The results show that the total content and composition of free amino acids in rhizoplane soil of tree species tested are quite different, and the amino acid content rhizospheric soil is lower than that of rhizosplane soil. The IAAcontent in rhizoplane and rhizospheric soils of tree species tested is in order of Robinia pseudoacacia>Sophora japonica >Populus beijingensis. After mixed with leguminous trees, the nutritional condition of Populus beijingensis is improved, its NPR is greatly increased, and SR is decreased.

Key words: Populus beijingensis, Robinia pseudoacacia, Sophora japonica, Biochemical compositions, Rhizospheric soil, Nutrition