## 4 Discussion  
### Variations inCO\({}_{2}\) Fluxes  
The NEE of the Middle Tien Shan grassland ecosystem showed an overall \"U\" curve change from April to September 2018. The ecosystem acted as a net carbon sink during the day and a net carbon source during the night. At 5:00, NEE showed a decreasing trend, while GPP showed an increasing trend due to the increase in solar height and radiation, leading to greater levels of photosynthesis than respiration in the grassland ecosystems [45]. This conclusion is consistent with the findings of previous research conducted by [PERSON] et al. in wetlands [18]. In terms of daily variation, NEE and GPP showed opposite trends, with the maximum CO\({}_{2}\) uptake occurring on June 17 at \(-9.73\) g C m\({}^{-2}\) d\({}^{-1}\). Similar findings were reported in a study of a rice ecosystem in the Khorgin grassland; the difference is that [PERSON] et al. found that in the rice ecosystem, the maximum CO\({}_{2}\) uptake occurs on August 15 th, with a value of \(-17.89\) g C\({}^{-2}\) d\({}^{-1}\)[19]. In terms of monthly variation, the overall cumulative CO\({}_{2}\) flux in the growing season of the Middle Tien Shan grassland ecosystem from April to September was \(-329.49\) g C m\({}^{-2}\), \(779.04\) g C m\({}^{-2}\), and \(449.55\) g C m\({}^{-2}\), respectively. Compared to the reported CO\({}_{2}\) flux of \(-183.45\) g C m\({}^{-2}\) in the growing season of the desert  
Figure 13: Average seasonal distribution of carbon flux in the Tien Shan region from April to September of 2018. (**a**) NEE, (**b**) GPP, and (**c**) Reco. ecosystem in the Gurbantunggut Desert by [PERSON] et al. [46], this indicates that the carbon sequestration capacity of grassland ecosystems in the arid region of northwest China is higher than that of desert ecosystems. Compared to other terrestrial ecosystems (Table 2), the Middle Tien Shan grassland ecosystem has a weaker carbon sink capacity than the forest [47] and meadow-rice ecosystems [19], but a stronger capacity than marsh and Siberian bog ecosystem [18; 48]. The hydrothermal conditions play a crucial role in affecting the strength of carbon sequestration [49]. The sandy grassland of Horqin has a substratum dominated by fine sand and clay-powder grains that strongly absorb solar radiation, resulting in an increased potential evapotranspiration, and combined with less precipitation; this leads to a stronger carbon sequestration capacity of the Middle Tien Shan grassland ecosystem compared to the sandy grassland ecosystem of Horqin [50]. In comparison to the grassland ecosystem of the Yunnan-Guizhou plateau [12], the carbon sequestration capacity of the Middle Tien Shan grassland ecosystem is weaker. The Yunnan-Guizhou plateau belongs to the subtropical climate zone with abundant water and heat conditions, while the Middle Tien Shan grasslands belong to the typical temperate continental climate with less precipitation and a dry climate. This weaker capacity of carbon sequestration in the Middle Tien Shan grassland ecosystem is due to the lack of water and heat conditions. The Relationship between CO\({}_{2}\) Fluxes and Meteorological Factors and Their Response to Temperature  
The RF model was used to calculate the correlation between CO\({}_{2}\) fluxes and meteorological factors in the Middle Tien Shan grassland ecosystem, and it was found that soil temperature was the main meteorological factor affecting CO\({}_{2}\) fluxes. As the soil temperature increased, the NEE of the Middle Tien Shan grassland ecosystem decreased, while GPP and Reco increased. This study is consistent with the previous conclusion that GPP and Reco increase significantly with global warming [51]. The carbon sink during the growing season occurs in June and July, which is consistent with favorable water and thermal conditions, despite the highest temperature occurring in August. This result may be attributed to the extreme drought climate in August, indicating that water and thermal conditions are important factors limiting photosynthesis in arid vegetation [52].