Simulation of buffering process and carbon sink effect of lime soil on sulfuric acid rain

After sulfuric acid rain settles to the surface and is buffered by lime soil, its participation in carbonate rock erosion and its impact on karst carbon sink are still unclear, which seriously restricts the accurate assessment of karst carbon sink effect in China. In order to clarify the buffering process and key control factors of lime soil to sulfuric acid rain, we conducted leaching experiments of sulfuric acid rain with pH=4.5 under different soil thicknesses. Results show that contents of Ca²⁺, Mg²⁺ and ${\rm{HCO}}_3^{-}$ in leaching solution decrease rapidly at the initial stage of leaching. When the leaching amount (after soil water reaches saturation) reaches 1,020 mL, the leaching loss of Ca²⁺, Mg²⁺ and ${\rm{HCO}}_3^{-}$ tends to be stable, and stable leaching amounts are 20 mg·L⁻¹, 6 mg·L⁻¹ and 40 mg·L⁻¹ respectively. The same ion in leaching solution of different thicknesses of soil columns shows the same trend, which indicates that the lime soil buffering of acid rain may mainly occur in the 10 cm (surface) soil layer. Ca²⁺ and Mg²⁺ exchanged by acid rain H⁺ in leaching solution only accounts for a small part, and the soil water-soluble Ca²⁺ and Mg²⁺ is the main part of Ca²⁺ and Mg²⁺ in leaching solution. In an open system, atmospheric and soil CO₂ dissolves in rainfall to form H₂CO₃, which not only increases carbon sink, but also exchanges Ca²⁺ and Mg²⁺ in soil with H+ generated by the dissociation of H₂CO₃. The leaching amount of Ca²⁺ and Mg²⁺as a result cannot be ignored. The special physical structure of topsoil may produce preferential flow, which will significantly reduce the exchangeable point of H⁺ in leachate, causing dissolution of carbonate minerals to assist the buffering of H⁺ in leachate. The karst area is the main place for carbon loss in terrestrial ecosystems, and the most intense carbon leaching occurs in surface soil. Exchangeable calcium, exchangeable magnesium and carbonate minerals in lime soil are principal reactants for buffering acid rain. The buffering capacity of exchangeable calcium and magnesium in lime soil with different thicknesses is greater than that of soil calcium carbonate minerals, and the former is 1.17-1.59 times as large as that of the latter. Under the same acidity and the same rainfall (after the soil water reaches saturation), the carbon sink generated by soil base ions participating in acid rain buffering is about 2.1 times as large as that generated by weathering buffering of carbonate minerals. Under the same rainfall conditions, the existence of lime soil can significantly increase carbon sink, and the carbon sink generated by lime soil with different thicknesses (≥10 cm) is roughly equal. According to this experiment and the rainfall data of Guilin, in the weathered residual soil area of pure limestone in Guilin (soil thickness greater than 10 cm), the participation of soil base ions in acid rain buffering can produce 0.59-0.93 mol·m⁻² carbon sink flux per year.