Abstract: By using stable carbon isotope technique, the leaf-level ¹³C discrimination was integrated to canopy-scale photosynthetic discrimination (Δ_canopy) through weighted the net CO₂ assimilation (A_net) of sunlit and shaded leaves and the stand leaf area index (L) in an A. mangium plantation, and the carbon isotope fluxes from photosynthesis and respiration as well as their net exchange flux were obtained. There was an obvious diurnal variation in Δ_canopy, being lower at dawn and at noon time (18.47‰ and 19.87‰, respectively) and the highest (21.21‰) at dusk. From the end of November to next May, the Δ_canopy had an increasing trend, with an annual average of (20.37±0.29)‰. The carbon isotope ratios of CO₂ from autotrophic respiration (excluding daytime foliar respiration) and heterotrophic respiration were respectively (-28.70±0.75) ‰ and (-26.75±1.3)‰ in average. The δ¹³C of nighttime ecosystem-respired CO₂ in May was the lowest (-30.14‰), while that in November was the highest (-28.01‰). The carbon isotope flux of CO₂ between A. mangium forest and atmosphere showed a midday peak of 178.5 and 217 μmol·m^-2·s^-1·‰ in May and July, with the daily average of 638.4 and 873.2 μmol·m^-2·s^-1·‰, respectively. The carbon isotope flux of CO₂ absorbed by canopy leaves was 1.6-2.5 times higher than that of CO₂ emitted from respiration, suggesting that a large sum of CO₂ was absorbed by A. mangium, which decreased the atmospheric CO₂ concentration and improved the environment.

Key words: Acacia mangium plantation, canopy photosynthetic discrimination, carbon isotope flux, soil fungi, N₂O production mechanism, N transformation processes, N₂O emission.