Research Article: Diurnal and Seasonal Variations in the Net Ecosystem CO2 Exchange of a Pasture in the Three-River Source Region of the Qinghai−Tibetan Plateau

Date Published: January 27, 2017

Publisher: Public Library of Science

Author(s): Bin Wang, Haiyan Jin, Qi Li, Dongdong Chen, Liang Zhao, Yanhong Tang, Tomomichi Kato, Song Gu, Xiujun Wang.


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.

Partial Text

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]. However, because the study of China’s grassland carbon flux began late, these studies have mainly focused on the low-lying regions of China [7].

We adopted eddy covariance to investigate the NEE for a single-sown cultivated pasture of Elymus nutans in the TRSR in 2008. Our results show that for the pasture, the NEE for the entire year was 140.01 g C m−2 year−1. Therefore, the cultivated grassland was a carbon sink during 2008. Because of the low temperatures in the TRSR, the annual Re of the pasture was only 403.57 g C m−2 year−1 in 2008, lower than those in most grassland ecosystems around the world. This finding implies that cultivated grassland establishment can both effectively resolve the grass−livestock conflict and properly improve the grassland carbon fixation capacity in the TRSR. Moreover, the Q10 value in the pasture for the entire growing season was 3.0, which was higher than that in low-elevation grassland ecosystems around the world, indicating greater sensitivity to elevated temperatures in the future in terms of ecosystem carbon loss in the study area. During the daytime, the NEE was primarily regulated by the PPFD; at night, the NEE was mainly regulated by Tsoil. A higher temperature can suppress photosynthesis in pastures, reducing the carbon absorption capacity of pasture ecosystems. The daily NEE and LAI were linearly related, and 32.5% of the NEE variation can be interpreted based on the LAI variation. Path analysis showed that the daily NEE in the growing season of cultivated grassland was controlled by various ecological factors at the same time. Among them, Tsoil was found to be the main decision-making factor for the daily NEE, and PPFD is the main constraining factor of the NEE in the studied ecological system.




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