Changes in surface sediment carbon compositions in response to tropical seagrass meadow restoration

Seagrass meadows are declining at a global scale, threatening their capacity as blue carbon sinks. Restoration of seagrasses (via seagrass seeds or plant transplantation) may recover their carbon sequestration capacity. Previous studies have predominantly focused on sediment organic carbon (SOC), while variations in sediment carbon compositions remain poorly understood, limiting our comprehension of the influence of seagrass restoration on sediment carbon stability. Here, we researched the differences in surface (0-3 cm) sediment carbon compositions in response to tropical seagrass transplantation among species (Thalassia hemprichii and Enhalus acoroides); specifically, differences in labile, recalcitrant and refractory SOC, as well as sediment inorganic carbon (SIC) compositions variations under transplanted T. hemprichii and E. acoroides communities. It was found that seagrass transplantation enhanced suspended particle organic matter, and epiphyte and macroalgae input to surface sediment, which recovered the surface SOC concentration and stock rapidly to natural levels (increased ∼1.6-fold) within two years following transplantation. The elevated contribution of epiphyte and macroalgae significantly increased the surface labile sediment organic matter (SOM), but not the recalcitrant and refractory SOM composition after short-term transplantation. Meanwhile, surface SIC was significantly elevated, which might be mainly ascribed to allochthonous carbonate particle trapped under transplanted area with implications for carbon sequestration. The higher canopy and longer leaf seagrass species, E. acoroides, had elevated SOC, SIC and was more labile composition, compared to T. hemprichii transplant. Overall, this research suggests that tropical seagrass transplantation can increase the surface SOC, SIC concentration by increasing the labile organic matter and allochthonous carbonate particle input, respectively, with varying significantly among seagrass species.