A Portable Method for Determining Nitrogen Efficiency of Woody Plants in the Field

A portable method for determining nitrogen efficiency of woody plants in the field. It includes the following steps: 1) Setting of nitrogen concentration gradient and blank control, 15N isotope labeled urea with 50% abundance; 3) Each plant is slowly watered along the base with 500-1000 mL of 15N labeled urea and blank control pure water with the above five concentration gradients; 4) After one week, the mature leaves at the top of each plant are collected; 5) The leaves are washed with pure water respectively, which are ground with a pulverizer after freeze-drying, and stored in a drying dish for determining; 6) The 15N abundance and total nitrogen content of the sample are determined by C / N element analyzer; 7) Calculating of the plant nitrogen efficiency; 8) Nonlinear fitting. By the method, the nitrogen efficiency of woody plants with different tree ages in the field can be directly determined.