Deep Coring Shows That Mangrove Sediments in Matang (Malaysia) Store up to Five Times More Carbon Than Previously Estimated

The carbon sequestration potential of mangroves, especially at sediment level, is known to be higher than other forest types. Sediment depth effects, however, have rarely been considered and carbon stock beyond 6 m depth has never been reported. The aim of this study is to account for deep sediment carbon storage to build a novel and more complete data set comprising all important ecosystem components, such as sediments, vegetation biomass, leaf litter and dead wood. For the first time, cores to a depth of 10 m were collected from the Matang Mangrove Forest Reserve (MMFR) in Malaysia. The 30-year silvicultural rotation, with two thinnings and a final clear-felling, enabled comparison of total carbon across different aged managed mangrove stands and the unmanaged. 3 protected forest, highlighting the impact of silviculture on carbon stocks. Our analysis suggests that the total carbon stock for the entire MMFR, when estimated to 10 m depth, is on the order of five times greater than earlier assessments, amounting to 107.34 Tg C. The carbon pool in the sediments was still around 100 Mg C ha^-1 at depths beyond 3-4 m, where most of the carbon was stored. Compared to the managed forest, the protected forest showed the highest sediment carbon pool (> 3000 Mg C ha^-1), proving that its unique set of biotic interactions, represented by flourishing burrowing crab communities and large amounts of decaying material on the sediment surface, allow the sediment to store at least 10% more carbon. On the other hand, the results from the managed stands showed the impact of clear-felling and thinning activities, with a loss of 456.7 and 284.8 Mg C ha^-1, respectively. Our deep coring approach complements the use of standard sampling techniques for mangrove sediment carbon estimation and highlights the importance of better assessments for future blue carbon projects worldwide.