## 4 Discussion  
### Carbon Budget Dynamics  
It is well known that the AGQTP has transferred from a carbon source to a carbon sink due to climate change [7; 17; 29]. However, the estimation of the carbon budget of the AGQTP remains uncertain [7; 29; 51]. This study simulated the carbon budget of the AGQTP using remote sensing data and models and validated the simulation results primarily using EC observation data. In addition, carbon budget data from multiple models in CMIP6 were used as a supplementary method to roughly validate the magnitude and overall trend of the simulated carbon budget in this study (Figure A1). According to the simulation results, the mean annual GPP and ER of the AGQTP were 330.9 Tg C yr\({}^{-1}\) and 315.9 Tg C yr\({}^{-1}\), respectively, between 2000 and 2020. The AGQTP acted as a carbon sink during this period, with a mean annual NEP of 15.25 Tg C yr\({}^{-1}\). This value was smaller than the estimation of 21.8 Tg C yr\({}^{-1}\) reported by [PERSON] et al. (2012) for the period of 2000-2009, but similar to the estimation of 16.76 Tg C yr\({}^{-1}\) reported by [PERSON] et al. (2015) for the period of 2000-2010 [7; 29]. The annual GPP, ER, and NEP increased at rates of 0.002 Pg C yr\({}^{-2}\), 0.013 Pg C yr\({}^{-2}\), and 0.0007 Pg C yr\({}^{-2}\), respectively, from 2000 to 2020 (Figure 3a-c), and their spatial distribution  
Figure 7: Comparison of trends in carbon budget indicators and climatic factors over sliding windows (**a**,**b**), and correlations between them (**c**) over the alpine grassland on the Qinghai–Tibet Plateau (AGQTP) during 2000–2020. Pslope_pw and Tslope_pw are the slopes of precipitation and temperature trends, respectively, in the preceding window relative to the carbon budget indicators.  
showed a general decreasing trend from southeast to northwest (Figure 3d-f), which was similar to that reported by previous studies [7; 29; 51]. The areas characterized as carbon sinks were mainly distributed in the southeastern and eastern regions, where the dominant ecosystem type was alpine meadow. Meanwhile, the areas with carbon sources were mainly located in the northwestern regions, where the dominant ecosystem type was alpine steppe (Figures 3f and 1a), and this spatial pattern of carbon sources and sinks was consistent with the findings of most previous studies [29; 31; 51].  
Although GPP, ER, and NEP increased in most alpine grasslands between 2000 and 2020, the proportion and spatial distribution of their trends exhibited notable variations (Figure 4). According to the spatial pattern of the trends (Figure 4a-c), the ER increased most significantly, followed by the GPP and NEP. The trend of NEP was jointly determined by the trends of GPP and ER. Therefore, the NEP showed a regional downward trend and a local significant decrease in the western QTP (Figure 4f) because the increase in GPP was less than the increase in ER in this region. The proportion of alpine grasslands with increased NEP generally decreased (Figure 4f), indicating that the increasing rate of carbon sequestration in the AGQTP slowed down during 2000-2020. However, future research is needed to verify whether this trend of slowdown will continue.