Significance of belowground production to the long-term carbon sequestration of intertidal seagrass beds

Abstract

The high biomass and sediment features of seagrass beds can make their belowground portions critical sources of blue carbon sinks. However, seagrass belowground production and decomposition have rarely been quantified in the field. To assess the significance of seagrass belowground production to carbon sequestration, belowground carbon budgets were constructed in intertidal seagrass beds of the late-successional species Thalassia hemprichii and the early-successional species Haloduleuninervis in southern Taiwan. For both species, the turnover rates of the belowground portions were much longer than that of the aboveground portion, so the belowground biomass was much higher than the aboveground biomass. The leaf productivity of both species was significantly higher than the belowground productivity, but most of the leaf production decomposed within a year. The lower turnover and slower decomposition rates of the belowground portions allowed the late-successional seagrass T. hemprichii to store more carbon in the sediments than the early-successional seagrass H. uninervis. Long-term changes for the past 20 years in the sediment depth showed that the sediments of seagrass beds were increasing in the habitats at low elevation but were decreasing or had no clear trends in the habitats at high elevation or on the windward side. The carbon storage rates according to the belowground production of T. hemprichii and H. uninervis were 0.3-4.7 and 1.5-2.3 g C m^-2 yr^-1, respectively, which can potentially contribute 53% of the long-term organic carbon storage in the low-elevation sediments.

Keywords: Blue carbon; Carbon budget; Carbon sink; H. uninervis; Thalassia hemprichii.