The nitrogen content of the flag leaf was determined by using amass spectrometer to analyze the N combustion at high tempera-tures during the determination of 13Csampleusing a stable isotoperatio mass spectrometer. The N content per unit of leaf dry masswas expressed as the N-mass (g N kg−1dry mass). The N contentper unit of leaf area was presented as N-area (g N m−2leaf area).N-area is the product of N-mass and SLM.