Diurnal and Seasonal Variations in the Net Ecosystem CO(2) Exchange of a Pasture in the Three-River Source Region of the Qinghai−Tibetan Plateau

Carbon dioxide (CO(2)) exchange between the atmosphere and grassland ecosystems is very important for the global carbon balance. To assess the CO(2) flux and its relationship to environmental factors, the eddy covariance method was used to evaluate the diurnal cycle and seasonal pattern of the net ecosystem CO(2) exchange (NEE) of a cultivated pasture in the Three-River Source Region (TRSR) on the Qinghai−Tibetan Plateau from January 1 to December 31, 2008. The diurnal variations in the NEE and ecosystem respiration (R(e)) during the growing season exhibited single-peak patterns, the maximum and minimum CO(2) uptake observed during the noon hours and night; and the maximum and minimum R(e) took place in the afternoon and early morning, respectively. The minimum hourly NEE rate and the maximum hourly R(e) rate were −7.89 and 5.03 μmol CO(2) m(−2) s(−1), respectively. The NEE and R(e) showed clear seasonal variations, with lower values in winter and higher values in the peak growth period. The highest daily values for C uptake and R(e) were observed on August 12 (−2.91 g C m(−2) d(−1)) and July 28 (5.04 g C m(−2) day(−1)), respectively. The annual total NEE and R(e) were −140.01 and 403.57 g C m(−2) year(−1), respectively. The apparent quantum yield (α) was −0.0275 μmol μmol(−1) for the entire growing period, and the α values for the pasture’s light response curve varied with the leaf area index (LAI), air temperature (T(a)), soil water content (SWC) and vapor pressure deficit (VPD). Piecewise regression results indicated that the optimum T(a) and VPD for the daytime NEE were 14.1°C and 0.65 kPa, respectively. The daytime NEE decreased with increasing SWC, and the temperature sensitivity of respiration (Q(10)) was 3.0 during the growing season, which was controlled by the SWC conditions. Path analysis suggested that the soil temperature at a depth of 5 cm (T(soil)) was the most important environmental factor affecting daily variations in NEE during the growing season, and the photosynthetic photon flux density (PPFD) was the major limiting factor for this cultivated pasture.