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The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species. Image 1 • Biogeochemical basis to understand organic preservation in seagrass soils. • Exceptional preservation of P. australis matter in seagrass soils over 1,700 years. • 75% of soil organic matter made of lignin, carbohydrate and non-pyrogenic aromatics. • Selective preservation of aged black carbon-like matter identified as non-pyrogenic aromatics. • Non-selective preservation of carbohydrate, lignin and protein with ageing. [ABSTRACT FROM AUTHOR]

Oyster Harbour, on the south coast of Western Australia, supports 3.6–3.9 km 2 of seagrass meadows, following the loss of approximately 2.8–3.1 km 2 in the 1980s. This small area of prevailing meadows hold significant carbon stores accumulated over the past 3000 years. In this study, we sampled three sediment cores from a Posidonia australis meadow and analysed organic matter (OM), organic carbon (OC) and inorganic carbon (IC) contents in bulk sediments, and δ 13 C signatures of OM. The OM and OC contents (mean ± SE) in the cores were 9.07 ± 0.36% and 2.24 ± 0.05%, respectively. The mean IC content was 3.16 ± 0.17%. δ 13 C signatures of the sedimentary OM ranged from −10.01‰ to −13.28‰. Using a Bayesian isotopic mixing model, it is estimated that 57–67% of the OM in the seagrass sediments was derived from P . australis detrital matter. The total carbon (TC) stores in 150 cm-thick seagrass sediments averaged 27.92 kg TC m −2 (10.79 kg OC m −2 and 17.13 kg IC m −2 ). Based on radiocarbon dating, the mean sediment accumulation rate was 0.0494 cm yr −1 , which led to a long-term TC accumulation rate of 8.92 g TC m −2 yr −1 (3.45 g OC m −2 yr −1 and 5.47 g IC m −2 yr −1 ). Based on historical seagrass cover (3.6–6.7 km 2 during the 1960s to 1980s), the estimated TC stores in 150 cm-thick seagrass sediments at Oyster Harbour would have been 101–187 Gg TC. The eutrophication-driven loss of seagrasses during the 1980s resulted in the absence of OC accumulation capacity amounting to 280–310 Mg OC (over 29 years). The loss of seagrass area could also have resulted in the release of 37–41 Gg CO 2 , assuming that all of the OC in shallow sediment is remineralised after meadow disturbance. These results exemplify the importance of seagrasses meadows as important carbon sinks and the potential for losses of carbon stores due to ecosystem degradation. [ABSTRACT FROM AUTHOR]