## 4 Discussion
### Driving Meteorological Factors of Carbon Budgets  
Alpine grassland ecosystems have been shown to be weak carbon sinks due to their special environments [2; 80], where strong solar radiation during the day favors photosynthesis by plants, whereas low temperatures at night inhibit autotrophic and heterotrophic respiration in the ecosystem.  
Past studies have shown that temperature and precipitation were the most important drivers of interannual variability in carbon fluxes in alpine grasslands [68; 71; 72; 81; 82; 83], and the carbon sink area was concentrated in the eastern part of the TRSR, which was attributed to the combination of water and heat conditions and high vegetation cover.  
The largest contribution of temperature to NEP in the TRSR was attributed to the earlier onset and faster rate of temperature increase in this region compared to other regions of China [15; 70; 84], which promoted the expansion of vegetation biomass and further led to more litter decomposition into the soil carbon pool. As a result, the carbon sink of alpine ecosystems increased [22; 71; 85].  
Through the analysis of meteorological data, it was found that the precipitation in the TRSR has increased in the past 20 years, especially in spring and winter [86]. The available evidence has indicated that water is one of the main controlling drivers affecting the carbon budget of alpine steppe ecosystems [87; 88; 89]. Our analysis found that the regions on which grassland NEP was significantly positively correlated with precipitation were concentrated in areas with precipitation levels below 500 mm in the TRSR, whereas in the south-eastern humid region, the correlation between precipitation and NEP did not reach a significant level (Figure 11h). Furthermore, some researchers have found that there was a time lag in the response of grassland ecosystems to precipitation [68], so we speculate that the increase in spring precipitation was also a significant cause for the increase in carbon sequestration in the TRSR.  
Solar radiation is the basic source of energy for photosynthesis in plants and directly affects their physiological and ecological mechanisms. The present study showed a significant negative correlation between solar radiation and NEP in some areas of the TRSR (20.71%). As precipitation is scarce in these areas, intense solar radiation exacerbates water evaporation, which negatively affects plant growth and NEP [90]. We also found that solar radiation positively and negatively contributed to NEP in 41.60% and 58.40% of the study area, respectively. Consistently, [PERSON] et al. [76] also found that solar radiation fluctuations affected the interannual variability of carbon fluxes. This intense solar radiation may have resulted in a substantial net CO\({}_{2}\) sink in these alpine grassland ecosystems. Freezing winters restrain the release of CO\({}_{2}\) and help ecosystems sequester carbon, meaning that the winter loss of CO\({}_{2}\) is nearly 1/3 of the summer CO\({}_{2}\) sequestration [30].  
As noted above, different carbon cycle models and analytical methods may lead to different effects and contributions of meteorological factors to carbon fluxes. In addition, considering the multiple effects of meteorological factors on carbon fluxes, more fine-scale field observations are imperative.