4.1 Reasons for profile variation of SIC content
In general, the variation of SOC content with soil depth after afforestation of arid desert areas mainly depends on the distribution of below-ground biomass (Schlesinger and Pilmanis, 1998). In contrast, the variation of SIC content is controlled by a variety of complex factors, such as soil properties, effective precipitation, soil moisture, and partial pressure of CO2, thus forming a complex profile distribution characteristic (Diaz-Hernandez et al., 2003). (Chang et al., 2012) reported that reforestation of the central Loess Plateau resulted in a redistribution of SIC along different soil layers. This study found that revegetation in sandy areas also caused redistribution of SIC content along the soil profile. The stratification characteristics of the SIC content with increasing stand age further corroborate this view. The accumulation process of carbonate, the main component of inorganic carbon, involves two main chemical reactions (Eqs. 4 and 5). On the one hand, there is an abiotic ”inorganic respiration” process in arid sandy soils, which absorbs CO2 from the atmosphere. Sufficient CO2can advance reactions (4) and (5) to the right to form loam-forming carbonate rocks. On the other hand, after planting vegetation in sandy areas, the root system releases large amounts of CO2into the soil by decomposing SOC, producing large amounts of free HCO3和H+ (Zamanian et al., 2016). The accumulated HCO3can drive reaction (5) to the right, and when Ca2+ is sufficient, carbonate can be precipitated to increase the accumulation of SIC (Li et al., 2012; Wang et al., 2015).
(4)
(5)
In this study, the SIC content of the 0 ~ 20 cm surface layer increased after vegetation restoration. The first reason is the increase in soil organic matter and soil microorganisms originating from dead fallen matter in the topsoil layer. During biological processes, the decomposition of soil organic matter and microbial activity increases the soil CO2 concentration. Subsequently, under adequate soil moisture conditions, driving equations (4) and (5) to the right, more abundant HCO3 is formed to bind with Ca2+ in the soil, facilitating the formation and precipitation process of carbonate. The second reason is the possible existence of the process of ”inorganic respiration” of the soil as described above, which leads to an increase in the SIC content of the soil surface. In the 20 ~ 100 cm soil layer, the SIC content further increased with the profile and showed a significant negative correlation with soil moisture. This result is consistent with the study on the SIC content of saline soils in the southern Gurbantunggut Desert (Wang et al., 2013). The possible reason for this is that in extremely arid areas, effective precipitation is more likely to affect the shallow soil SIC content. The increased CO2 in the soil is dissolved at shallow depths and follows the effective precipitation downward. This process continuously reacts chemically with shallow Ca2+ and Mg2+, which causes the migration and precipitation of carbonates. In the 120 ~ 240 cm soil profile, the variation of SIC content was mainly influenced by root biomass, CO2 partial pressure Ca2+ and HCO3. With the increase of stand age, the deep soil root biomass is also increasing and growing deeper into the soil. On the one hand, the increased input of root litter stimulates the activity of soil microorganisms and accelerates the decomposition of the litter. During decomposition SOC is mineralized to produce more CO2, driving equation (4) to the right, further dissolving in the soil solution to form HCO3-, which subsequently combines with Ca2+ released from decomposing litter to precipitate as CaCO3 (Zhao et al., 2016). On the other hand, in extremely arid soil environments, secondary carbonates can form in the mucilaginous sheaths around root hairs. The root system is enriched with unused excess Ca2+ and a large amount of HCO3 accumulates in the mucilaginous sheath due to respiration, allowing the mucilaginous sheath to provide a unique environment for Ca2+ and HCO3 binding. Under these two effects, the formation of secondary carbonates in deep soils is promoted (Monger et al., 2015). (Liu et al., 2014)studied the profile change characteristics of SIC in agricultural fields, and grasslands restored for 12 years and 22 years of restoration, and finally found that SIC storage decreases with revegetation. This is in contrast to the results of this study where SIC content increased with the vegetation restoration sequence. Some soil carbonates may be temporarily decomposed to CO2 due to the decrease in soil pH and the increase in soil moisture after the restoration of grassland on agricultural land. In deep soils from 240 to 300 cm, the SIC content was significantly and positively correlated with soil pH. Increasing stand age causes decreasing SIC content, which is due to the large amount of organic acid secreted by the deep root system, which makes the pH decrease. Under the conditions of relatively low Ca2+ and HCO3 content, the large enrichment of CO2 from SOC decomposition and root respiration drives equation (5) to the left, leading to carbonate dissolution and lower inorganic carbon content (Jin et al., 2018). In previous studies, soil samples were collected at depths ranging from 20 cm (Li et al., 2021) to 300 cm (Wang et al., 2010), and SIC profile changes exhibited different responses to different soil layer combinations. These results indicated that after afforestation of sandy areas in arid and semi-arid regions, SIC content was mainly influenced by aboveground litter decomposition, effective precipitation, and CO2 partial pressure in the shallow layer; and by soil moisture, root litter and pH value in the deep layer.