## 4 Discussion
### Climate-Driven Mechanisms of the Carbon Budget
According to the results of the sliding window analysis, although the positive driving of precipitation on the carbon sink (NEP) was strengthened, the negative effect of temperature on the carbon sink was also strengthened (Figure 3). In addition, according to the fitted line of precipitation and temperature (Figure 1b), relative to 2000, precipitation increased by about 0.45% per year, while temperature increased by about 1.82% per year, which was much greater than the precipitation increase rate. We believe that this difference in temperature and precipitation changes is an important explanation for the slowdown in the increase rate of grassland carbon sinks from 2000 to 2020. Secondly, there was significant spatial heterogeneity in the impact of climatic factors on the carbon budget of the AGQTP. Precipitation was the primary climatic driving factor for GPP and NEP in most alpine grasslands located in semi-arid and arid climate zones (Figure 6a,c,e), but it was the restraining factor for ER in most of the grasslands. Water deficiency and droughts in arid and semi-arid climate zones can limit photosynthesis because soil moisture, as an intermediary, is crucial for plants to absorb and transport nutrients [59; 60; 61]. In addition, water availability has important effects on soil nutrient availability [59; 62]. Therefore, precipitation variation determined the carbon uptake and sequestration dynamics of the AGQTP by altering photosynthesis and soil nutrients. The impact of precipitation on ER is relatively complex [56; 63]. The ER was tightly coupled to the photosynthetic rate [64], indicating that precipitation can increase ER through increasing photosynthesis. However, precipitation can also reduce the temperature and the air permeability of the soil, thereby inhibiting respiration [63]. In the western QTP, temperature was the primary driver of respiration (Figure 6d), and precipitation led to lower temperatures, resulting in lower ER. Compared to precipitation, temperature was the primary climatic driving factor of the ER in most alpine grasslands on the QTP (Figure 6d), as temperature acted as the primary limiting factor for ER on the cold QTP [55; 56; 65]. The influence of temperature on GPP and NEP had relatively higher spatial heterogeneity (Figure 6b,d,f). Although the GPP in most alpine grasslands (64%) was primarily driven by temperature, these grasslands were mainly located in the eastern part of the semi-arid climate zone and semi-humid zone with relatively higher precipitation. However, warming in relatively dry regions such as the arid zone and the western part of the semi-arid zone suppressed the increase in GPP (Figure 6b). In relatively humid areas, temperature is the primary limiting factor for plant growth, and an increase in temperature will lead to an increase in GPP. However, in relatively arid regions, water is the primary limiting factor, and rising temperature will exacerbate water loss and drought, ultimately inhibiting grassland growth. The effect of temperature on NEP is more complex and spatially heterogeneous [7] because NEP is jointly affected by GPP and ER, which respond differently to temperature, as described above. The NEP in most alpine grasslands on the QTP was reduced by temperature, mainly in the semi-arid zone and arid zone where warming decreased GPP and increased ER. The downward trend in NEP in the west of the QTP (Figure 4c) was attributed to the greater increase in temperature compared to precipitation (Figure 6), leading to a higher contribution of temperature to ER than precipitation to GPP. This resulted in a slower increase in the GPP than ER and eventually led to a decrease or even a significant decrease in NEP. In conclusion, the temperature driving of GPP and NEP was limited by local water availability. In general, temperature increases promoted carbon absorption and sequestration only in areas and during periods with sufficient water.