![]() ![]() However, identifying the creditable GHG offset benefit requires isolating seagrass-enhanced C org sequestration over time 18, accounting for sequestered C org turnover 19, and determining whether seagrass presence also increases GHG emissions of CH 4, N 2O, and CO 2 evasion associated with CaCO 3 buried in seagrass sediment, all of which would reduce the GHG benefit from seagrass-enhanced carbon sequestration 20, 21, 22. 3 provide, perhaps, the closest approximation, a net GHG removal estimate for natural seagrass meadows based on carbon flux measurements and a one-time sediment C org burial rate. However, despite increasing interest in seagrass ‘blue carbon’ and studies reporting seagrass sediment C org stocks 5, 13, 14, 15, including several from restored meadows 13, 16, 17, a study has yet to quantify the net greenhouse gas (GHG) removal from the atmosphere resulting from a seagrass restoration project 18. Seagrass restoration transfers C org back to the sediment 9, 12, 13. Global meadow loss may, therefore, release 50–330 Tg CO 2 yr −1 back to the atmosphere 11. Seagrass bed erosion following meadow collapse accelerates oxidation and remineralization of this sediment C org 8, 9, 10. Seagrass meadows potentially contain 4,200-8,400 Tg C org in bed sediments and an additional 151 Tg C org in above- and belowground biomass 5-a significant global carbon stock threatened by accelerating seagrass habitat loss from coastal development, eutrophication, climate change, and other anthropogenic impacts 6, 7. Seagrass meadows have been identified as important sinks in the global carbon cycle, because they are highly productive systems that bury organic carbon (C org) 1, 2, 3, 4. Managers should also consider seagrass co-benefits, which provide additional incentives for seagrass restoration. The financial benefit for this highly successful project, $87 K at $10 MtCO 2e −1, defrays ~10% of the restoration cost. ![]() Despite tripling the N 2O flux to 0.06 g m −2 yr −1 and increasing CH 4 8-fold to 0.8 g m −2 yr −1, the meadow now offsets 0.42 tCO 2e ha −1 yr −1, which is roughly equivalent to the seagrass sequestration rate for GHG inventory accounting but lower than the rates for temperate and tropical forests. Restoring seagrass removed 9,600 tCO 2 from the atmosphere over 15 years but also enhanced both CH 4 and N 2O production, releasing 950 tCO 2e. We measured all of the necessary GHG accounting parameters in the 7-km 2 Zostera marina (eelgrass) meadow in Virginia, U.S.A., part of the largest, most cost-effective meadow restoration to date, to provide the first seagrass offset finance test-of-concept. However, no study has quantified net GHG removal from the atmosphere from a seagrass restoration project, which would require coupled C org stock and GHG flux enhancement measurements, or determined whether the creditable offset benefit can finance the restoration. Awarding CO 2 offset credits may incentivize seagrass restoration projects and help reverse greenhouse gas (GHG) emissions from global seagrass loss. ![]()
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