4.3 The importance of deep SIC
Most previous studies on SIC have focused on the upper layers, especially the top 1.0 m. However, the excavation of deeper soil profiles becomes more important for the study of SIC storage (Mi et al., 2008; NEPSTAD et al., 1994; Veldkamp et al., 2003). (Li et al., 2007) estimated that the SIC storage from 1 m depth to intact soil is about 57% in sandy soils and 61% in gray desert soils. (Wang et al., 2010) reported that more than 50% of the SIC in forests, scrub, grasslands, and sandy areas in temperate regions of northern China were located at a depth of 100 ~ 300 cm. (Wang et al., 2013) found stratified storage of SIC at 0 ~ 1 m, 1 ~ 3 m, 3 ~ 6 m, and 6 ~ 9 m depth in saline and alkaline soils in arid northwest China. SIC storage below 1 m is above 80%, and SIC storage below 3 m is above 50%. The values estimated in this study were higher than theirs, and we found more than 60% of SIC storage in the 1m to 3m soil profiles in sandy and post-afforestation areas. The differences between these values may be due to different soil sampling depths and vegetation types. Compared to the estimated total SIC storage in the top 1 m of the soil of 53.3 Pg, soils below 1 m in the arid desert areas of northwest China still contain 6.2 Pg or more of SIC storage (Mi et al., 2008). Recent studies have shown that carbon uptake in saline soils in arid zones can be as high as 62-622 g C m -2 y−1. Thus, on a global scale, the deep soil SIC pool and its variability may be more critical than we have recognized. In China, 47% of the land is arid and semi-arid, and the deeper layers of the soil are rich in SIC. The contribution of deep soil SIC storage to total soil carbon storage after vegetation construction has not been well studied for these areas.
Understanding the changes in deep-section SIC storage is essential for assessing the climate change mitigation potential of soil carbon pools(George et al., 2012; Jobbagy and Jackson, 2000). In this study, we analyzed the soil profile variation of SIC storage along the age of artificial H. ammodendron plantation and its influencing factors. On the one hand, the variation characteristics of SIC storage in different forest age stages provide important data for regional carbon estimation. On the other hand, understanding the effect of vegetation restoration on changes in SIC storage under different soil depth conditions helps us to understand the importance of deep SIC for soil carbon pool estimation. Our results suggest that considering only shallow SIC when assessing the impact of regional vegetation restoration on SIC reserves will result in a significant underestimation of the SIC pool. However, all soil samples are difficult to reach down to 300 cm due to a large amount of labor and cost consumption. In this context, the combination of remote sensing techniques and model simulations for in-depth soil sample collection in typical areas is a feasible method for estimating the soil inorganic carbon pool.