Abstract: A field experiment was conducted to investigate the effects of topdressing nitrogen on the concentrations and storage of major nutrients and their internal and external fluxes in  Eucalyptus urophylla × E. grandis  leaves. The mass loss of the fallen leaves was 26%, and thus, to characterize the nutrients concentration by per unit mass was not correspondent with reality, while the nutrients concentration per unit area was a correct characterization. Comparing with those in living leaves, the ash and organic matter contents in fallen leaves decreased by 33% and 25%, respectively, suggesting that some hydrolyzable organic matter was transferred back to the trees before leaf fallen. The nitrogen use efficiency of eucalyptus leaves was up to 44%, showing the luxury absorption of N. The trunk and branch recovered more than 70% of N, P, and K in fallen leaves, indicating that the three elements were mainly cycled in internal flux. Calcium was mainly in external flux, with the scale being about 90%. The internal fluxes of Mg, Si, and ash were almost the same, and their external flux scale was slightly higher than their internal flux scale.

Key words: Black earth, CH₄ emission, Microbial biomass, Soil respiration