4 Discussion
4.1 The carbon sequestration potential of vegetation restoration in dryland area
In this study, we found that planted vegetation in arid areas showed a carbon sink not only in the vegetation establishment period in the first 20 years but also in the established stable status. EVI showed a gradual increase during the first 10–20 years of vegetation restoration, with large variation in the following years (Figure 1). During the stable period (since 2009), here is an increasing trend in NEP (by 5.65 gC m-2 yr-1 yr-1, p < 0.05) and an annual increasing trend of 6.17%, associated with a significant increasing annual GEP (8.63 gC m-2 yr-1 yr-1, p<0.05) and slightly increasing annual Re (3.0 gC m -2 yr-1 yr-1, p > 0.05) (Figure 4). The NEP and GEP exhibited a significant increasing trend only in the spring season (p<0.05, Figures 5). Previous field survey results have shown that desertification is too complex an issue to be curbed by shrub restoration. However, shrub restoration can halt soil and vegetation degradation and increase ecosystem carbon stocks (Yang et al., 2014; Huang et al., 2014; Chen et al., 2018; Wang et al., 2020). Results from a sand fixation project in a humid area showed that the average annual carbon sequestration in the first 10 years of sand fixation vegetation establishment (from 2000 to 2010) was 158 gC m-2 yr-1 (Lu et al., 2018), which was higher than the results from our arid artificial vegetation sand fixation area. Annual carbon sequestration rates in artificial shrub ecosystems from the southern edge of the semi-arid Mu Us Desert vary over a range of 77~-76 gC m-2 yr−1 (Jia et al., 2016; Liu et al., 2019). The observed NEP of restoration vegetation of this study in arid regions is higher than the NEP in semi-arid regions, even though the GEP in semi-arid regions is higher than in arid regions. This is mainly due to the higher Re in semi-arid regions (Liu et al., 2019).
The annual amount of carbon sequestered by different ecosystems varies considerably from region to region in natural shrub ecosystems. Piao et al. (2009) found that they are also an essential carbon sink in China, with an average carbon sequestration of 10 ± 5 gC m-2 yr-1. The saline shrub community in the Gurbantunggut desert (arid land) showed a small carbon sink with annual carbon sequestration of 26 ± 13gC m-2 yr-1 (Liu et al., 2016). The arid shrub ecosystem near Baja California, Mexico, was found to be a carbon source from measured results in 2002–2008, with annual carbon sequestration of -79 ± 117 gC m-2 yr-1 (Bell et al., 2012). The variability of carbon sink rates observed in the Mojave Desert of the USA and the scrub desert of Mexico ranges from 40 to 120 gC m-2 yr-1 (Jasoni et al., 2005; Biederman et al., 2017). Similar evidence was observed in our 13 years of observation.
The carbon fixation capacity of shrub planting vegetation in the Shapotou area (Figure S1) is higher than that of natural shrub vegetation. The inter-annual variability of carbon flux was always a carbon sink in wet and dry years (Figure 4). This indicates that the planting of artificial shrubs could potentially enhance the carbon sink function in desert areas. It is worthwhile to highlight that opposing views on the role of restoration vegetation on carbon sequestration have been reported (Moomaw et al., 2019) as young trees sequestered more carbon than old stands (Pugh et al., 2019). Further studies under different climate and environmental conditions are imperative to clarify the role of restoration vegetation on carbon sequestration function.