Decomposition Rates of Surficial and Buried Organic Matter and the Lability of Soil Carbon Stocks Across a Large Tropical Seagrass Landscape

The paradigm for understanding the accumulation of organic carbon in vegetated coastal “blue carbon” habitats holds that burial of organic carbon (Corg) slows decomposition and leads to stability of carbon stocks. Further, it is generally assumed that the presence of the plant communities contributes to the buried organic matter and the stability of the carbon stocks. In this study, these assumptions were tested and the lability of soil organic carbon was examined as a function of environmental and plant community drivers. Samples of surficial sediment and seagrass community characteristics were collected at 93 locations across the ca. 15,000 km2 of seagrass beds in south Florida. Ramped pyrolysis was used to describe the relative lability of soil organic carbon across the landscape. Organic matter (OM) was lost at all temperatures from 180 to 600 °C, suggesting that even the relatively high combustion temperature of 550 °C underestimates OM content by ≈ 10% on average. Additionally, deployments of model substrates (canvas strips) were used to examine decomposition rates of buried and surficial organic material at a subset of these sites. On average, finer, muddier soils contained slightly higher Corg stocks than coarser sediment sites, but the relationships between sediment grain size and seagrass community structure was weak. The lability of soil organic carbon varied with sediment grain size; as much as 80% of the Corg was refractory in coarse-grained soils compared with less than 30% in muddy soils. In muddy soils, burial decreased cellulose decomposition rate by an average of 22–39% compared with surficial breakdown, but in coarse-grained soils, burial enhanced cellulose decomposition rate by at least 55%. Taken as a whole, this study suggests that burial does not enhance Corg storage in all blue carbon environments, and that soil C stores are only weakly correlated with seagrass biomass at the landscape scale.