A potential CO2 carrier to improve the utilization of HCO3- by plant-soil ecosystem for carbon sink enhancement.

Introduction: Improving the rhizospheric HCO₃^- utilization of plant-soil ecosystem could increase the carbon sink effect of terrestrial ecosystem. However, to avoid its physiological stress on the crop growth, the dosage of HCO₃^- allowed to add into the rhizosphere soil was always low (i.e., <5-20 mol/m³).

Objectives: To facilitate the utilization of relatively high concentrations of HCO₃^- by plants in the pursuit of achieving terrestrial carbon sink enhancement.

Methods: In this study, the feasibility of directly supplementing a high concentration HCO₃^- carried by the biogas slurry to the plant rhizosphere was investigated using the tomato as a model plant.

Results: The CO₂-rich biogas slurry was verified as a potential CO₂ carrier to increase the rhizospheric HCO₃^- concentration to 36 mol/m³ without causing a physiological stress. About 88.3 % of HCO₃^- carried by biogas slurry was successfully fixed by tomato-soil ecosystem, in which 43.8 % of HCO₃^- was assimilated by tomato roots for the metabolism, 0.5 ‰ of HCO₃^- was used by microorganisms for substances synthesis of cell structure through dark fixation, and 44.4 % of HCO₃^- was retained in the soil. The rest of HCO₃^- (∼11.7 %) might escape into the atmosphere through the reaction with H⁺. Correspondingly, the carbon fixation of tomato-soil ecosystem increased by 150.1 g-CO₂/m²-soil during a tomato growth cycle. As for the global countries that would adopt the strategy proposed in this study to cultivate the tomato, an extra carbon sink of soil with about 1031.1 kt-C per year (i.e., an additional 0.21 tons of carbon per hectare soil) could be obtained.

Conclusion: This would be consistent with the goal of soil carbon sink enhancement launched at COP21. Furthermore, the regions with low GDP per capita may easily achieve a high reduction potential of CO₂ emissions from the agricultural land after adopting the irrigation of CO₂-rich biogas slurry.