Seagrass Blue Carbon – what is it made of and how well is it sequestered?

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the biogeochemical processes that allow the preservation of OC in seagrass sediments remain... [ view full abstract ]

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the biogeochemical processes that allow the preservation of OC in seagrass sediments remain unclear. Nor is it clear whether this stored sediment is likely to be re-mineralised following disturbance of seagrass sediments. In this study, we sampled sediments of a Posidonia australis meadow in Oyster Harbour (Western Australia) to investigate the preservation of sedimentary OC. We then exposed the sediments to oxygenated conditions to determine potential rate so remineralisation. A 133 cm long sediment core was taken from the meadow and radiocarbon dated. The organic matter from 10 discreet depths was characterised using solid state CP/MAS 13C NMR to examine the preservation of organic matter down the sediment profile (i.e. with age). There were minimal changes in organic composition over 1900 years of accumulation, indicating the consistent dominance of seagrasses at the site and the long-term preservation of the buried organic matter. Seagrass-derived organic matter (i.e. lignin, carbohydrate and a black carbon-like matter) constituted 70-83% of total sedimentary organic matter, the remainder being mostly proteins. The depth profiles of the dominant types of OC indicate different types of preservation mechanisms: selective preservation for the black carbon-like matter and non-selective preservation for carbohydrates and lignin. The findings demonstrate the exceptional preservation of seagrass-derived OC, leading to its long-term storage in the seagrass soils. There was no detectable loss of organic carbon following exposure of the deep sediments to air for 9 months. This indicates that even following disturbance, the OC stored preserved by seagrass meadows may remain recalcitrant and therefore a permanent sink of atmospheric CO2.