2.2 Tissue assays
The dried plant samples were determined at Nankai University. To determine N and P concentrations in plant leaves, the leaf samples were first digested by the concentrated H2SO4-H2O2method (Bao, 2000). After digestion, the solution was diluted and precipitated, and the supernatant was collected. N content was determined by SKD-800 Kjeldahl nitrogen analyzer, and P content was analyzed by the molybdenum-antimony anticolorimetry method (Bao, 2000). The analysis of each sample was repeated three times. The concentrations of N and P in soil were determined in the same manner as those in leaves.