Soil carbon stocks, lability and decomposition rates of surficial and buried organic matter in a large tropical seagrass landscape

Our paradigm for understanding the accumulation of organic carbon in blue carbon habitats holds that burial of organic carbon slows decomposition and leads to stability of carbon stocks. Further, we generally assume that the presence of the plant communities contributes to the buried organic matter and the stability of the carbon stocks. In this study, we tested these assumptions and also examined the relationship between the lability of organic carbon as a function of environmental and plant community drivers. We sampled surficial sediment and seagrass community characteristics at 45 locations located across the ca. 15,000 km2 of seagrass beds in south Florida. We used ramp pyrolysis to describe the relative lability of soil organic carbon across the landscape. Additionally, we deployed model substrates and examined decomposition rates at a subsection of these sites, with deployments on the soil surface and buried in the rhizosphere at each site. We found that finer, muddier soils contained higher C_org stocks that coarser sediment sites, and that the relationships between sediment grain size and seagrass community structure was weak. The organic carbon from muddy sediments was more labile than carbon found in coarse-grained soils. In muddy soils, burial decreases cellulose decomposition rate, but in coarse grained soils, burial enhances cellulose decomposition rate. Taken as a whole, our data suggests that burial does not enhance C_org storage in all blue carbon environments, and that soil C stores are only weakly correlated with seagrass biomass at the landscape scale.