Fertilization Changes Seagrass Community Structure but not Blue Carbon Storage: Results from a 30-Year Field Experiment

Seagrass ecosystems are attracting attention as potentially important tools for carbon (C) sequestration, comparable to those terrestrial and aquatic ecosystems already incorporated into climate change mitigation frameworks. Despite the relatively low C stocks in living biomass, the soil organic carbon pools beneath seagrass meadows can be substantial. We tested the relationship between soil C storage and seagrass community biomass, productivity, and species composition by revisiting meadows experimentally altered by 30 years of consistent nutrient fertilization provided by roosting birds. While the benthos beneath experimental perches has maintained dense, Halodule wrightii-dominated communities compared to the sparse Thalassia testudinum-dominated communities at control sites, there were no significant differences in soil organic carbon stocks in the top 15 cm. Although there were differences in delta C-13 of the dominant seagrass species at control and treatment sites, there was no difference in soil delta C-13 between treatments. Averages for soil organic carbon content (2.57 +/- 0.08 %) and delta C-13 (-12.0 +/- 0.3 aEuro degrees) were comparable to global averages for seagrass ecosystems; however, our findings question the relevance of local-scale seagrass species composition or density to soil organic carbon pools in some environmental contexts.