## 4 Discussion
### Variations inCO\({}_{2}\) Fluxes
In August, grassland ecosystems were susceptible to high-temperature stress due to enhanced solar height and solar radiation, which, together with low precipitation, accelerated the shortening of the grassland phenological cycle, leading to increased potential evapotranspiration and respiration in the grasslands. Enhanced evapotranspiration could lead to water stress in plants [53], resulting in weaker CO\({}_{2}\) flux uptake and release in grassland ecosystems, as well as weaker GPP and Reco. Previous studies have shown that drought or high temperatures cause the CO\({}_{2}\) balance of arid and semi-arid ecosystems to shift from carbon sink to carbon source [54; 55; 56; 57]. The grassland ecosystem had the strongest carbon sequestration capacity when the soil temperature is 25 \({}^{\circ}\)C. Above this temperature, the grassland ecosystem is prone to heat stress, while below this temperature, it is susceptible to cold stress. The current investigation confirms this conclusion, finding that when soil temperature rises above 35 \({}^{\circ}\)C, extreme \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Study Period** & **Position** & **CO\({}_{2}\) (g C m\({}^{-2}\))** & **Reference** \\ \hline Grassland & April 2018–September 2018 & 43\({}^{\circ}\)28\({}^{\prime}\)N, 87\({}^{\circ}\)12\({}^{\prime}\)E & \(-\)329.49 & This study \\ Grassland & July 2017–August 2018 & 27\({}^{\circ}\)46\({}^{\prime}\)N, 107\({}^{\circ}\)28\({}^{\prime}\)E & \(-\)425.14 \({}^{\star}\) & [12] \\ Sandy grassland & May 2015–September 2015 & 42\({}^{\circ}\)55\({}^{\prime}\)N, 12\({}^{\circ}\)42\({}^{\prime}\)E & \(-\)120.54 & [50] \\ Siberian bog & May 2015–August 2015 & 60.90\({}^{\circ}\)N, 68.70\({}^{\circ}\)E & \(-\)202 & [48] \\ Floating blanket marsh & January 2016–December 2016 & 25\({}^{\circ}\)07\({}^{\prime}\)N, 98\({}^{\circ}\)33\({}^{\prime}\)E & \(-\)233.8 \({}^{\star}\) & [18] \\ Forest & January 2015–December 2015 & 30\({}^{\circ}\)06\({}^{\prime}\)N, 78\({}^{\circ}\)12\({}^{\prime}\)E & \(-\)526.87 \({}^{\star}\) & [47] \\ Meadow—rice & May 2020–October 2020 & 43\({}^{\circ}\)20\({}^{\prime}\)N, 12\({}^{\circ}\)37\({}^{\prime}\)E & \(-\)769.24 & [19] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of CO\({}_{2}\) fluxes in different types of terrestrial ecosystems (values marked with \({}^{\star}\) indicate that the study time scale is one year). drought conditions are highly likely to occur. Drought and high temperatures lead to dehydration of grass cells, and the lack of water and heat conditions adversely affect the photosynthesis of grassland [58]. When the soil temperature exceeds 10 \({}^{\circ}\)C, the Reco of grassland ecosystems begins to increase. As the soil temperature surpasses 40 \({}^{\circ}\)C, Reco reaches its maximum value. Air temperature also had a strong correlation with carbon fluxes in the grassland ecosystems, which was consistent with previous studies [59; 60; 61]. In our research, we have discovered that the temperature threshold for the occurrence of carbon sinks in grassland ecosystems is 15 \({}^{\circ}\)C. This further demonstrates that vegetation requires the most suitable temperature for growth through photosynthesis [62].