title: Diurnal and Seasonal Variations in the Net Ecosystem CO2 Exchange of a Pasture in the Three-River Source Region of the Qinghai-Tibetan Plateau.
abstract: Carbon dioxide (CO2) exchange between the atmosphere and grassland ecosystems is very important for the global carbon balance. To assess the CO2 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 CO2 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 (Re) during the growing season exhibited single-peak patterns, the maximum and minimum CO2 uptake observed during the noon hours and night; and the maximum and minimum Re took place in the afternoon and early morning, respectively. The minimum hourly NEE rate and the maximum hourly Re rate were -7.89 and 5.03 μmol CO2 m-2 s-1, respectively. The NEE and Re 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 Re 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 Re 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 (Ta), soil water content (SWC) and vapor pressure deficit (VPD). Piecewise regression results indicated that the optimum Ta 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 (Q10) 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 (Tsoil) 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.
authors.affiliation: Key Laboratory of Marine Ecosystem and Biogeochemistry, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China.|College of Life Science and Agriculture and Forestry, Qiqihar University, Qiqihar, China.
authors.forename: Bin
authors.initials: B
authors.lastname: Wang
authors.affiliation: Key Laboratory of Marine Ecosystem and Biogeochemistry, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, China.
authors.forename: Haiyan
authors.initials: H
authors.lastname: Jin
authors.affiliation: Northwest Plateau Institute of Biology, The Chinese Academy of Science, Xining, China.
authors.forename: Qi
authors.initials: Q
authors.lastname: Li
authors.affiliation: Northwest Plateau Institute of Biology, The Chinese Academy of Science, Xining, China.
authors.forename: Dongdong
authors.initials: D
authors.lastname: Chen
authors.affiliation: Northwest Plateau Institute of Biology, The Chinese Academy of Science, Xining, China.
authors.forename: Liang
authors.initials: L
authors.lastname: Zhao
authors.affiliation: National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.
authors.forename: Yanhong
authors.initials: Y
authors.lastname: Tang
authors.affiliation: Doctoral Program in Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
authors.forename: Tomomichi
authors.initials: T
authors.lastname: Kato
authors.affiliation: College of Life Science, Nankai University, Tianjin, China.
authors.forename: Song
authors.initials: S
authors.lastname: Gu
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism*; D017753:Ecosystem; D065948:Grassland; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology; D045483:Rivers; D012621:Seasons; D013696:Temperature; D018609:Tibet
chemical_list: D002245:Carbon Dioxide; D002244:Carbon