| Based on the eddy covariance (EC) and gradient data of both Xilinhot National Climate Observatory during2009-2011and Shouxian National Climate Observatory during2008-2010, this paper calculated the CO2flux of the two ecosystems, summarized its variation features and analyzed the relationship between the CO2flux and meteorological elements (temperature, moisture, photosynthetically active radiation) and boundary layer characteristics. The results are summarized as follows:(1)During the observation, both the grasslands in Xilinhaote and farmland in Shouxian are CO2sinks. CO2flux of grassland in Xilinhaote, reaching up to about120g/m2every year, is smaller than in the farmland of Shouxian where the CO2flux is-700g/m2. The top daily CO2flux in Xilinhaote is-8g m-2d-1while in Shouxian it is is-12g m-2d-1.(2)The annual variation of CO2flux in Xilinhaote grassland is significant with great difference existing in not only the intensity of the CO2flux, but also in the peak time of CO2flux. During the three years, the maximum CO2flux is achieved in2011, followed by2010and2009in order. Regarding the time distribution, CO2flux has only one absorption peak in May2009and two absorption peak values in mid June, late July and early August2010. Also, there are two absorption peaks, respectively in mid May and August2011. The farmland of Shouxian is more stable, the peak time of CO2sinks appears in April and August respectively in2010and2009, but the NEE value is higher in2010than in2009.(3) The daily variation in the two places is single peak-shaped, but both are distinctly different. The daily maximum value of CO2flux in Xilinhaote appears at10o’clock in the morning, earlier than the value of farmland in Shouxian, which is acquired at noon and is symmetrical essentially. In addition, the diurnal variation of CO2flux of farmland in Shouxian is more dramatic than that of grassland in Xilinhaote with the diurnal variation amplitude reaching up to1.4mg m-2s-1while the variation amplitude in Xilinhaote is only0.25mg m-2s-1.(4) CO2flux is strongly affected by the vertical turbulence intensity. If turbulence is intense, the CO2flux is bigger whereas the CO2vertical transmission is weaker. CO2flux is almost negative in unstable conditions (Z/L<0). CO2flux concentrates in the range of Z/L<0.5. With the increase of the absolute value of stability, the CO2flux decreases. The neutral condition is the best atmosphere conducive to CO2vertical transmission. It is beneficial for crops to absorb CO2if the turbulent exchange coefficient is big, but this relation is not linear. The carbon flux does not change significantly when the coefficient is very big.(5) Precipitation, photosynthetic active radiation, temperature and soil moisture are important factors affecting the CO2flux of Xilinhaote, but the key factors responsible for the significant difference in the three years are precipitation and soil moisture. The difference of precipitation and soil moisture in Shouxian County are the factors causing difference of the first peak of CO2flux in2009and2010, while temperature and photosynthetically active radiation is the reason resulting in the difference of the second peak of the two years.,(6) The sensible heat flux of Xilinhaote is about-70-450W/m2during the three years. Its maximum value in Shouxian County is300W/m2in June and ranges within-50-200W/m2e in other periods. The latent heat flux is in the range of0-300W/m2in Xilintaote, while in Shouxian County it is0-450W/m2. In general, the latent heat flux is larger than the sensible heat flux on Shouxian County farmland, and in the Xilinhaote grassland, the situation is just the opposite.(7) In summer, CO2flux, latent heat flux and sensible heat flux are in significantly negative correlation on Shouxian farmland. CO2flux and latent heat flux, sensible heat flux are nonlinear correlated on Xilinhaote grassland, and the correlation coefficient is smaller than that of Shouxian County. The energy balance degree of Shouxian is from large to small in spring, autumn, summer and winter. There are overestimates on turbulent fluxes in four seasons on Xilinhaote grassland, and it is bigger in winter and spring than in summer and autumn. The energy closure degree over grassland is the biggest, followed by wheat, and the smallest over rice. It is because the calculation ignores the canopy energy storage Q, resulting in the.reduced energy ratio. Imbalance of energy is a common phenomenon in flux observation. It is generally believed that the degree of imbalance is in the range of10%-30%. Energy balance degree of Shouxian County and Xilinhaote National are in the range, which illustrates the eddy correlation method is reliable in the ecosystem flux observation of Shouxian County and Xilinhaote grassland.. |
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