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.