Abstract: Background, aim, and scope The inorganic carbon balance process of CO2(g)-CO2(aq) -(aq)-CaCO3(s) is prevalent in saline-alkali soil, and its dynamic process dominates the inorganic carbon exchange of the ground-gas interface, which also controls the fixation and leaching of the soil inorganic carbon. Soil dissolved inorganic carbon (SDIC) is an active dynamic carbon, which is an important mechanism of soil interception of inorganic carbon. In the current study of soil inorganic carbon, it has been concentrated in non-salt soils like farmland, plains, plateau, deserts soil carbonate content and carbon reserves estimation and different ways of land use on soil inorganic carbon content. There are few studies on inorganic carbon in saline soil, and mainly concentrated in the soil of single salinization in Xinjiang region, but not much in the northern saline-alkali soil inorganic carbon. The study site is located in Hetao Irrigation District of Inner Mongolia, due to its special geographical location and climatic conditions, the soil salinization degree is high and the salinization area is large. In order to explore the saline soils in arid areas with different soil soluble salt content on the dynamic distribution characteristics of inorganic carbon, select 7 kinds of saline soil to study the profile distribution of soil dissolved inorganic carbon and base ions using the method of random points in this area. Materials and methods The soil samples ware collected at a distance of close to the bare ground with different EC by the soil auger in depth from 0 — 100 cm in late August 2016. The soil samples of the same soil layer were collected three times, and the soil samples obtained from the soil samples were fully mixed, and the soil samples were obtained with 42 soil samples in total. The litter on surface of the ground has been removed before sampling. Samples are dried under a natural ventilation indoor, and the grits with the diameter more than 2 mm are removed as well as roots and debris. The treated soil is used to determine soil physicochemical properties, soil salt content, soil dissolved inorganic carbon and soil organic matter. Results (1) The increase of dissolved inorganic carbon content in soil with different salinity is decreasing. As the depth of soil increases, the content of dissolved inorganic carbon decreases first and then increases, which is low in the shallow layer of 0 — 50 cm, and the content of deep 50 — 100 cm is accumulated. (2) The content of soil salinity with different salt content increases with the increase of conductivity. As the depth of soil deepens, the content of salt-base ions decreases gradually, and it has strong table cohesion. The composition of soil salt-base ions in the study area was composed of Ca2+ and mainly, with an average content of 27% and 29% respectively. The content of K+ and Mg2+ is relatively small, with the average content accounting for 13% and 7% of the ions. (3) Correlation analysis showed that soil dissolved inorganic carbon content and EC has significant negative correlation, and no significant correlated to the pH, base ions average content and EC has significantly positively correlation, and no significant correlated to the pH. Discussion (1) It can enhance that leach action of soil under certain condition of salinity, and accelerate the leaching of dissolved inorganic carbon in the soil. The solubility of the saline-alkali soil solution to CO2 is high. In CO2(g)-CO2(aq)-HCO3(aq)-CaCO3(s) inorganic carbon equilibrium, the higher the CO2 concentration, the more the balance will be moved to the right to form more SDIC. Due to the presence of desalt in the near-surface soil, the water will dissolve some dissolved carbonate and bring it underground. The carbonate solution will gradually accumulate in the process of downward migration, and the dissolved carbonate will be trapped in the vertical direction, resulting in the distribution difference in the vertical direction. It was found that that content of 0 — 50 cm soil lay in the 0 — 100 cm cross section was lower than the content of 50 — 100 cm soil lay, means the SDIC mainly distributed in the lower layer of soil. (2) The soil electrical conductivity can reflect the content of the mixed salt of the soil. The higher the EC, the higher the concentration of soluble ions in the soil, the greater the total salt content. The soil in arid and semi-arid regions has high salinity and strong secondary salinization. Due to high temperature and low rainfall, the surface evaporation is very strong, and the large amount of soluble salt ions carried in the upward migration of groundwater will accumulate at the surface of the soil, resulting in a shallow distribution of exchangeable salt-base ions. Conclusions Combining results from dissolved inorganic carbon content and salt-base ions content in different saline-alkali soil can provide information about the effect of soil saline-alkali level is huge. The profile distribution of dissolved inorganic carbon content and salt-base ions in different soil depths shows the existence and migration mechanism of them in soil. Recommendations and perspectives In arid and semi-arid areas of saline soil, inorganic carbon cycle has the potential of carbon sequestration, the dissolution of carbonate-reprecipitation process affects soil carbon fixation and transferring, fully understand the distribution characteristics of the inorganic carbon in saline soil, which is the key to explore the soil environment quality and the aggregation of materials in arid regions.
Keywords: Salinization soil; Soil dissolved inorganic carbon; Base ions; Profile distribution