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.
Highlights
Grassland ecosystems occupy approximately one-third of the total global land area and form an important component of the earth’s carbon circulation [1]
During the past few decades, ecologists have studied the effects of environmental factors, biological factors and management measures on the carbon exchange between the land surface and the atmosphere of the grassland ecosystem by using eddy covariance [2, 3], and these ecologists have noted the significance of human activity on the carbon exchange process [4, 5]
Our results show that for the pasture, the net ecosystem CO2 exchange (NEE) for the entire year was 140.01 g C m−2 year−1
Summary
Grassland ecosystems occupy approximately one-third of the total global land area and form an important component of the earth’s carbon circulation [1]. During the past few decades, ecologists have studied the effects of environmental factors (such as radiation, temperature, water and soil nutrition), biological factors and management measures on the carbon exchange between the land surface and the atmosphere of the grassland ecosystem by using eddy covariance [2, 3], and these ecologists have noted the significance of human activity on the carbon exchange process [4, 5]. The grassland of China occupies approximately 40% of the nation’s total land area and plays an extremely important role in the regional circulation of carbon [6]. There have been reports on the process of carbon exchange between the land surface and the atmosphere and on the carbon exchange mechanisms of the primary natural vegetation types (e.g., alpine meadows and alpine shrubs) over the last several years [10, 11], there have only been a few reports on the carbon exchange process, the source/sink function of planted vegetation (e.g., cultivated grassland) and the mechanisms controlling the exchange among environmental and biological factors
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