Abstract: The measurement system of Li-8100 carbon flux and the modified assimilation chamber were used to study the photosynthetic characteristics of cotton (Gossypium hirsutum L.) canopy in the oasis edge region in middle reach of Heihe River Basin, mid Hexi Corridor of Gansu. At the experimental site, soil respiration and evaporation rates were significantly higher in late June than in early August, and the diurnal variation of canopy photosynthetic rate showed single-peak type. The photosynthetic rate was significantly higher (P＜0.01) in late June than in early August, with the daily average value being (43.11±1.26) μmol CO₂·m^-2·s^-1 and (24.53±0.60) μmol CO₂·m^-2·s^-1, respectively. The diurnal variation of canopy transpiration rate also presented single-peak type,with the daily average value in late June and early August being (3.10±0.34)mmol H₂O·m^-2·s^-1 and (1.60±0.26) mmol H₂O·m^-2·s^-1, respectively, and differed significantly(P＜0.01). The daily average value of canopy water use efficiency in late June and early August was (15.67±1.77) mmol CO₂·mol^-1 H₂O and (23.08±5.54) mmol CO₂·mol^-1 H₂O, respectively, but the difference was not significant (P＞0.05). Both in late June and in early August, the canopy photosynthetic rate was positively correlated with air temperature, PAR, and soil moisture content, suggesting that there was no midday depression of photosynthesis in the two periods. In August, the canopy photosynthetic rate and transpiration rate decreased significantly, because of the lower soil moisture content and leaf senescence, but the canopy water use efficiency had no significant decrease.

Key words: cotton, canopy photosynthetic rate, Heihe River, oasis, temperate steppe, nitrogen addition, phosphorus addition, aboveground biomass.