Evaluation of atmospheric CO2 sequestration by alkaline soils through simultaneous enhanced carbonation and biomass production

- Gypsum application increased inorganic C but decreased organic C in alkaline soils.
- pH reduction allowed plant growth to produce biomass and increased soil organic C.
- Root exudates tended to acidify soil and inhibited pedogenic carbonation.
- Plant-soil system (Na-tolerant species) had a CO2 sequestering capacity of 93 t/ha.
- Implications for cost-benefit analysis of alkaline sodic soil reclamation projects

A series of microcosm experiments were conducted. The objectives were to evaluate the effects of Ca/Mg-bearing materials on CO2 sequestration in highly alkaline sodic soils (Sodosol) through carbonation and biomass production. Application of gypsum resulted in an increase in inorganic carbon and a decrease in organic carbon. The addition of talc did not significantly enhance carbonate formation. Soluble CaCl2 and MgCl2 did not have significantly better effects on soil carbonation, as compared to gypsum. The one-year growth experiment using five widely cultivated pasture grasses revealed that accumulation of carbonates following gypsum application could be inhibited by plant growth; the organic acids secreted from plant roots were likely to facilitate soil carbonate dissolution. In comparison with pedogenic carbonation, carbon sequestration by biomass production was much more evident. However, the biomass carbon gain varied markedly among the five species with Digitaria eriantha showing the highest biomass carbon gain. This further enhanced the accumulation of soil organic carbon. At the end of the experiment, an estimated CO2 sequestering capacity of 93 t/ha was achieved. The research findings have implications for cost-benefit analysis of alkaline soil reclamation projects.