The annual NEP was correlated to the DCUP, but not to the annual temperature and precipitation (Figure 6). However, during individual seasons, temperature or precipitation were well-correlated with the NEP (Figure 7). In spring, temperatures portrayed a non-significant increasing trend (Figure 2a), and the increased spring temperature resulted in a significant advance in the SCUP (Figures 3 and S3). This is consistent with the findings of Fu et al. (2017), where SCUP was mainly influenced by spring temperature. Both were significantly correlated with NEP in spring (Figures 7a and 7b). The increase in spring temperature and earlier SCUP led to a gradual increase in NEP in spring (Figure 5). Spring temperature caused a significant increase in GEP but not in Re (Figure 7a). At the same time, earlier SCUP had a significant effect on both GEP and Re (Figure 7b). This may be due to an increase in early vegetation activity due to a warm spring (Wolf et al., 2016), and an earlier spring SCUP (Fu et al., 2017), which increases ecosystem productivity. In desert ecosystems, the pattern of change in Re is determined by soil respiration (Gao et al., 2015). Due to the limitation of soil water, increasing temperature leads to a decrease in the rate of soil respiration (Guan et al., 2021), therefore, no significant correlation was observed between increasing temperature and Re in spring. The NEP in spring was not affected by spring rainfall (Figure S3). This may be because the vegetation activity and carbon balance in spring were affected by the legacy effects of the previous year's precipitation (Sala et al., 2012). The soil moisture from late autumn of the previous year to the following spring (Jia et al., 2016) may reduce the response of spring NEP to the current season's precipitation. This suggests that temperature increases and phenological changes in spring may be more essential for spring and year-round NEP in desert artificial vegetation areas. Liu et al. (2019) also found a similar knot in a semiarid shrubland.
The sum of the mean values of summer and autumn NEP amounts was 92.22 gC m -2, which exceeded 90% of the total annual NEP. Hence, the summer and autumn net carbon uptake made the largest contribution to the annual net carbon uptake. It was not temperature (Figure S3) but summer precipitation that determined the amount of NEP in summer and autumn (Figure 7c and d). Although the ECUP was significantly earlier in autumn (Figure 3), it was not due to autumn temperatures (Figure S3). ECUP is influenced by other factors such as soil moisture content and radiation, in addition to temperature (Fu et al. 2017). Neither autumn temperature nor ECUP had a significant effect on NEP (Figure S3).
Our work has demonstrated that after 30 years of artificial vegetation establishment in the desert, the ecosystem carbon sequestration rate still maintained an increasing trend (Figure 4). Long-term studies have shown that after several decades of succession, the sand-binding vegetation protective system has evolved from a landscape dominated by mobile sand dunes to an artificial-natural composite ecosystem dominated by shrubs, herbaceous plants, and biological soil crusts (Li et al., 2004a, 2014). This has enabled the sand-fixing vegetation system to maintain stable function and structure (Li et al., 2005; 2013). The increase in soil carbon and nitrogen storage during vegetation restoration processes (Li et al., 2022) has provided a foundation for the continuous increase in primary productivity (Luo, Field, & Jackson. 2006). This may promote an increase in NEP. The increased carbon sequestration in the artificial vegetation of the desert did not lead to more water loss through ET and further drying of soil moisture (Figure S4). This mechanism diagram reflects the impact of vegetation restoration system and climate on NEP (Figure 8) . Our results suggest that appropriate vegetation restoration in arid areas can increase ecosystem carbon sequestration over longer timescales and mitigate climate change, with relatively low environmental consequences and risks. Considering the vast area of degraded land in the global drylands, the carbon sequestration effect of this model should be given more attention.