Subsurface organic ameliorant is beneficial in reducing inorganic carbon loss and improving carbon sequestration in saline soils

Subsurface organic ameliorant is an appropriate practice to improve soil quality and enhancing crop yield in saline soils. However, its influence on soil inorganic carbon (SIC) stock and interactions between SIC, soil organic carbon (SOC), and total carbon (C) stock remains unclear. A three-year field experiment (2020–2022) was thus conducted to investigate how surface and subsurface organic ameliorants (SA and SSA) chemically and biologically regulate SIC sequestration in saline soil (electric conductivity >1000 μS cm⁻¹). After organic ameliorant for three years, 0–45 cm SIC stock under all treatments decreased by 1.0–4.5 t ha⁻¹. However, SIC stock was higher under SSA by 7 % as compared to non-amended controls (CK) at 15–30 cm. This was attributed to increased Ca²⁺, Mg²⁺, microbial (bacterial and fungal) richness resulting from reduced Cl⁻ concentration and higher moisture, and suppressed CO₂ emissions. Beyond the 15–30 cm soil, CO₂ emissions and microbial richness also negatively and positively affected 0–15 cm SIC stock, respectively, while Mg²⁺ positively influenced 30–45 cm SIC stock. Furthermore, a negative correlation existed between SOC and SIC stock across 0–45 cm, contrasting with a positive correlation within 15–30 cm. The contribution of SIC to total C stock progressively increased with soil depths. These findings demonstrate that subsurface organic ameliorant can mitigate SIC loss and enhance total C sequestration in saline ecosystems.