Determination of blue carbon sequestration in seagrass sediments over climatic time scales (>100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.
Valuing sedimentary ‘blue carbon’ stocks of seagrass meadows requires exclusion of allochthonous recalcitrant forms of carbon, such as black carbon (BC). Regression models constructed across a Southeast Asian tropical estuary predicted that carbon stocks within the sandy meadows of coastal embayments would support a modest but not insignificant amount of BC. We tested the prediction across three coastal meadows of the same region: one patchy meadow located close to a major urban centre and two continuous meadows contained in separate open embayments of a rural marine park; all differed in fetch and species. The BC/total organic carbon (TOC) fractions in the urban and rural meadows with small canopies were more than double the predicted amounts, 28 ± 1.6% and 36 ± 1.5% (±95% confidence intervals), respectively. The fraction in the rural large-canopy meadow remained comparable to the other two meadows, 26 ± 4.9% (±95% confidence intervals) but was half the amount predicted, likely owing to confounding of the model. The relatively high BC/TOC fractions were explained by variability across sites of BC atmospheric supply, an increase in loss of seagrass litter close to the exposed edges of meadows and sediment resuspension across the dispersed patchy meadow.
The capacity of wetlands to mitigate greenhouse gas (GHG) emissions is the sum of two services: the protection of vulnerable organic stocks from remineralisation, and the capacity to sequester GHGs relative to their anthropogenic replacements. Organic carbon accumulation (CA) down through the sediment column is often taken as the measure of sequestration because of its capacity to record long-term variability and trends. However, we demonstrate that: i) CA is not equivalent to sequestration as net ecosystem production (NEP) for open systems; it requires the subtraction of the initial deposition rate of labile allochthonous carbon sources; ii) CA also requires subtraction of intrinsically allochthonous recalcitrants down through the sediment column, and together with subtraction of autochthonous recalcitrants from organic stock services; iii) CA as a climatic mitigation service also requires a diagenetic correction, as the annual deposition of labile organic carbon continues to remineralise over the long-term; and iv) preserving of a wetland has a significantly greater mitigation potential than restoring one. To address the above concerns, a global diagenetic solution is proposed, applied and tested for a tropical seagrass and mangrove. As expected, traditional CA estimates were disproportionately larger than their respective diagenetically modelled NEPs, and together with stocks fell within the ranges reported in the literature, with a final carbon accreditation highly dependent on the choice of their anthropogenic replacements. The review demonstrates that mitigation concepts and measurements for natural carbon sequestration solutions require re-evaluation to avoid GHG emissions above their capacity or reduce the ability to fulfil emission targets.
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