Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Abstract

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.

Inorganic and black carbon hotspots constrain blue carbon mitigation services across tropical seagrass and temperate tidal marshes

“…Traditionally, this has been assumed to have largely ceased near the surface after 1 or 2 years of deposition (Cebrian, 2002). However, this assumption has recently been challenged There is evidence to suggest that mineralization within mangroves sediments continues after years and that 50% of the TOC, excluding BC, may be lost from seagrass sediment horizons within a years of deposition (Chuan et al, 2020;Maher et al, 2017).…”

“…However, whiting events after the loss of seagrass canopies have not yet been reported. Nevertheless, such events could conceivably increase the water columns pCO 2 during wind resuspension from the loss of a seagrass canopy (Gallagher et al, 2020 in press).…”

“…Traditional assessments have failed to remove allochthonous organic recalcitrants from the equation (Chew and Gallagher, 2018;Chuan et al, 2020;Gallagher, 2014;Gallagher, 2015;Gallagher, 2017;Gallagher et al, 2020 in press;Gallagher et al, 2019). These stable organic forms are not produced by the blue carbon ecosystem and deposition by the ecosystem does not afford additional protection from remineralization.…”

Translating seagrass science into action

“…In the first two papers, Rahman and Yaakub (2020) and Chuan et al (2020) assess some of the ecosystem services provided by seagrass meadows. Rahman and Yaakub engage with the challenging task of determining both the ecosystemservice and socio-economic value of seagrass meadows at Mukim Tanjung Kupang, Johor, Malaysia.…”

“…Although they calculate the habitat value at US$57 731.80 ha À1 year À1 , they consider this a substantial underestimate because of the large number of seagrass ecosystem-services for which there is currently no value available. Chuan et al (2020) focus on blue carbon, testing current sequestration assumptions by 500-day anoxic decomposition and mineralisation experiments. Their data and models suggest that carbon sequestration may be lower than previously thought, and they highlight the need for greater certainty regarding model assumptions that are key to understanding how seagrasses can be part of a solution to climate change.…”