Experimental insights into the stability of karst carbon sink by submerged macrophytes

Karst regions occupy 15–20% of the Earth’s continental area, and the quantity of atmospheric CO₂ trapped by surface karstification is approximately 0.61 Gt C/year. However, the stability of the karst carbon sink remains questioned. Here, we focused on two typical submerged macrophytes (H. verticillata and M. spicatum) collected from a karst spring-fed river to explore their influences on the stability of karst carbon sink. The pH drift technique reveals that both H. verticillata and M. spicatum collected from the karst spring-fed river can use HCO₃⁻ as a carbon source, providing direct evidences of the karst carbon sink sequestrated by submerged macrophytes. In the DIC enrichment water, the mean RGR fresh weights are 0.092 day⁻¹ and 0.047 day⁻¹ for H. verticillata and M. spicatum, respectively, which are 5.38 and 10.23 times compared with that in the control group. Taking the morphological traits to indicate their biomass allocation strategies, we find that for both species the accumulated biomass is allocated to apical elongation and root production, the difference to lateral tillering for H. verticillata as well. Overall, H. verticillata has a relatively higher ΔBiomass:ΔHCO₃⁻ ratio than M. spicatum, this is in agreement with the results of pH drift and morphological traits. Moving forward, we establish a model ( \(\Delta Biomass=0.0985\times \Delta pH+0.0969\) $\mathrm{\Delta }Biomass=0.0985\times \mathrm{\Delta }pH+0.0969$ , P < 0.001) based on the experimental data so that can be applied to quantitatively estimate the karst carbon sequestrated by submerged macrophytes in karst spring-fed rivers. Considering submerged macrophyte is the cornerstone in biomass pyramid, our work highlights that submerged macrophytes act as a carbon pump in karst aquatic ecosystems.