Exogenous calcium-induced carbonate formation to increase carbon sequestration in coastal saline-alkali soil

Promoting soil carbon sequestration is a possible way to mitigate global warming. To investigate the effects of exogenous calcium on soil carbon sequestration during the application of organic matter to improve coastal saline-alkali soil. In this study, a 30-day incubation experiment was based on the application of corn straw biochar + chicken manure (BM) and rice straw + chicken manure (SM). Usages of exogenous calcium in each treatment under each organic matter combination as follow: CK (No exogenous calcium), CaSi1 (1.24 g CaSiO₃, i.e. 4.28 g Ca kg^-1 soil), CaSi2 (2.48 g CaSiO₃, i.e. 8.56 g Ca kg^-1 soil), CaOH1 (0.79 g Ca(OH)₂, i.e. 4.28 g Ca kg^-1 soil), CaOH2 (1.58 g Ca(OH)₂, i.e. 8.56 g Ca kg^-1 soil), CaSiOH (1.24 g CaSiO₃ + 0.79 g Ca(OH)₂, i.e. 8.56 g Ca kg^-1 soil). Results showed that exogenous calcium significantly reduced CO₂ emission. Organic matter addition promoted the loss of SOC, and exogenous did not significantly affect the mineralization of SOC albeit strongly increased SIC, making up for the loss of SOC, increasing soil total carbon and realizing soil carbon fixation. Soil carbon fixation was mainly realized by the reaction of exogenous calcium with CO₂ generated by mineralization and converting it into calcium carbonate. pH and soil CO₂ emission are the major controlling factors for soil inorganic carbon sequestration. Therefore, applying organic matter with exogenous calcium can realize soil carbon fixation by generation of calcium carbonate.