4.2 Effect of vegetation restoration on inorganic carbon stock
Although there are more studies on the effects of revegetation on SIC (He et al., 2016; Li et al., 2016; Wang et al., 2013), there are no clear conclusions yet. A part of the study found that SIC reserves remained unchanged after revegetation, which was attributed to carbonate leaching in the upper part of the profile and precipitation in the deeper part of the profile (Jin et al., 2014; Monger et al., 2015; Wang et al., 2016). Another part of the study found an increase in SIC attributed to the involvement of exotic Ca2+ and Mg2+ in the formation of SIC (BOETTINGER and SOUTHARD, 1991; Zhao et al., 2016) , the formation of carbonate from biogenic CO2 (He et al., 2016), or a decrease in soil erosion (Fu et al., 2009). The study also found a decrease in SIC, which was attributed to the transfer of SIC into groundwater with water infiltration (Kessler and Harvey, 2001), or in low pH soil environments, SIC is released into the atmosphere as CO2 (Liu et al., 2014). The seemingly contradictory results reflect the uncertainty of the changes in SIC storage after revegetation.
In this study, the SIC storage in the sand showed an increasing trend after afforestation. (Su et al., 2010) reported that planting camphor pine and aspen in the Badangirin sands promoted the accumulation of SIC. (Gao et al., 2017) found that SIC storage also increased with stand age sequence after planting aspen (8 ~ 30 years old aspen) in Mauwusu sands, with significantly higher SIC content after vegetation restoration than in moving sands. The findings of this study are consistent with those of the aforementioned scholars on SIC storage after the afforestation of sandy areas. The increase in SIC after the afforestation of sandy areas is due to the high input of litter and the high rate of decomposition of organic matter, resulting in the release of large amounts of CO2 into the soil (Zhang et al., 2013). A considerable amount of CO2 is involved in the chemical reaction of formula (4), forming a large amount of HCO3, which leads to the precipitation of loam-forming carbonates (Zamanian et al., 2016). In addition, the trapping and deposition of fine particles in the wind and sand streams by the plant canopy, after the afforestation of sandy areas also contributes to the accumulation of SIC. Soiling fine particles contain a large amount of carbonate, which leads to rapid accumulation of SIC (Chang et al., 2012). However, our results differ from earlier ones on the Loess Plateau of China and the Colorado Plateau Desert, USA (Chang et al., 2012; Sartori et al., 2007). In their previous findings, vegetation restoration reduced SIC storage. The reduction in SIC storage may be caused by surface runoff, and irrigation in these areas, which could reduce dissolved SIC content from the restored vegetation-soil ecosystem. In arid semi-arid sands are not affected by irrigation or heavy precipitation. Therefore, our results suggest that vegetation construction has the potential to increase SIC storage in arid semi-arid sands.
The establishment of vegetation on shifting sands at different ages also provided us with a good-time series to study CSR. This study shows that CSR is not a constant value; instead, it tends to decrease with increasing stand age. The CSR of short afforestation time (<5 years) is greater than that of a long time (>5 years). If we classify the H. ammodendron plantation from 5 to 46 years old as young, near-mature, mature, and over-mature forests, respectively. We found that the CSR was greatest in young stands and least in over-mature stands after moving sand afforestation. Therefore, if the change of CSR over time after vegetation restoration is neglected, the magnitude of long-term changes in SIC storage after sand afforestation will be incorrectly estimated, which will greatly increase the uncertainty of soil carbon estimation. Currently, many studies have only estimated the increase or decrease in SIC storage after afforestation (An et al., 2019; Raza et al., 2020), while few studies have quantified how the change of CSR with afforestation time (Chen et al., 2018; Wang et al., 2019)(Chen et al.,2018; Wang et al.,2019)[34-35][34,35][34, 35]. Therefore, the CSR in these studies was assumed to be the same over 30 years, with the result that SIC storage increments with stand age were overestimated. To improve the accuracy of CSR calculations, we strongly recommend long-term monitoring of SIC at different stages in long-term studies or using space-for-time methods.