Dissolved inorganic carbon input significantly lowers carbon dioxide flux but not methane flux in shallow macrophyte-dominated systems with positive effects on carbon stocks

Abstract

Background

With the increase in the inorganic carbon input from watersheds, elevated dissolved inorganic carbon (DIC) concentrations will significantly impact the carbon cycle in freshwater ecosystems. Moreover, the limited diffusion rate of CO₂ in water, coupled with the lack of functional stomata, greatly restricts the ability of submerged macrophytes to absorb CO₂ from their aquatic environment. The importance of bicarbonate (HCO₃⁻) for submerged macrophytes becomes more pronounced. Current research focuses on the effects of DIC (notably HCO₃⁻) on the phenotypic plasticity of submerged macrophytes, while its impact on their carbon stock capabilities has rarely been reported.

Results

In this study, Myriophyllum spicatum served as the model macrophyte within a mesocosm experimental system to assess the impact of HCO₃⁻ enrichment (0.5 to 2.5 mmol L⁻¹) on carbon stocks and emissions across a one-year period. Our findings indicated that the addition of HCO₃⁻ had a non-significant inhibitory effect on the diffusive fluxes of methane (CH₄) emissions. Concurrently, it significantly reduced CO₂ fluxes within the systems. The annual average CO₂ fluxes across the four HCO₃⁻ addition levels were -3.48 ± 7.60, -6.78 ± 5.87, -7.15 ± 8.68, and -14.04 ± 14.39 mol m⁻² yr⁻¹, respectively, showing significant differences between low /medium- and high- HCO₃⁻ addition levels.

Conclusion

The addition of HCO₃⁻ enhanced carbon stocks in water, macrophytes and the entire system, with minimal effects on carbon sedimentation stocks. Our study provides valuable insights into understanding the carbon sink capacity of aquatic ecosystems and elucidates the underlying mechanisms driving these processes on a system scale.