Coastal Landforms and Accumulation of Mangrove Peat Increase Carbon Sequestration and Storage

Many studies have highlighted the considerable belowground carbon storage of mangroves and other coastal ecosystems (as much 30% of total ocean carbon storage). Mangroves are among the most carbon-rich forests in the tropics, containing on average more than 1,000 Mg C/ha. We sampled mangrove sediments in four locations along the Pacific Coast of Mexico, from the Baja California Sur in the north to Chiapas near the Guatemalan boarder. These sites varied in their coastal geomorphology and rainfall regimes. The mangroves of rainy Chiapas possessed the deepest and most carbon-rich Rhizophora peat deposits of any of the sites (in places more than 2,000 Mg/ha). More surprisingly, in Balandra, one of the desert mangrove lagoons of Baja California Sur, the ­Avicennia-dominated mudflat zone of the forest possessed deep and rich peat deposits, ranging from 400-1,300 Mg/ha. This forest, hemmed in by relatively steep hillsides demonstrates the potential for mangroves to accrete carbon-rich peat vertically when local topography precludes their landwards expansion with sea-level rise. Our microscopic examination of root fibers from these peat deposits revealed the importance of Avicennia to the formation of buried organic matter deposits. We used ¹⁴C dating to track the age of the Baja California deposits, whose ages ranged between 1193 and 1636 BP. Plotting the calibrated ¹⁴C age of each peat sample from Balandra against the depth of the sample below the mean sea-level, we found a very significant linear trend (r² = 0.87, P < 0.0001) with a slope of 0.070 ±0.007 mm/yr. Belowground carbon sequestration rates during recent decades varied from very low (ca. 0.1 Mg.ha^‑1.yr^‑1) in a receding fringe in Bahía Magdalena or a halophilic hinterland in Balandra, to 9–20 Mg.ha^‑1.yr^‑1 in a Rhizophora mudflat in La Encrucijada. With only 0.49% of the total area, the mangroves around the Gulf of California store 18% of the total belowground carbon pool of the whole region, 76 Tg in total.