Mangrove forests, pantropical coastal wetland ecosystems, provide important ecosystem services including fisheries nursery habitat provision, coastline protection, nutrient cycling, and other local benefits. Of global import is their ability to store carbon in organic peat belowground for hundreds to thousands of years, with more carbon per unit area than any other forest type. Mangrove carbon storage relies in part on the high primary productivity of these systems, but essential to the large and long-lived nature of this storage is the slow decomposition of buried mangrove peat. In this study, we explore the relationship between carbon content and macronutrient concentration in the sediments and plot the slow course of buried mangrove detritus over time. In three mangrove sites with deep (< 1 m) deposits of peat in the area of Bahia de La Paz, B.C.S., Mexico, we cored the sediments until rejection with a Russian peat corer, and from these cores obtained 5 cm samples at 20 cm intervals. In these samples we measured total carbon, organic carbon, nitrogen, and phosphorus, as well as 14C age.
We observed high percentage carbon by mass (14 ± 6%) and high C:N ratios (29 ± 7) in peat samples. Radiocarbon dates allowed us to reconstruct the accumulation and slow decomposition of organic matter over the last 850 ± 15 years. In a survey of sites in the area, forests with deep peat deposits are associated with high content of clay in overlying sediments. The materials composing the peat deposits (primarily mangrove roots) and their tree species of origin are also investigated. Limitations on microbial decomposition by microbes, whether due to low oxygen and nutrient concentrations or the recalcitrance of the peat material, likely help explain this slow decomposition. These results shed light on the processes maintaining these slow rates, and further work will disentangle the roles and relative importance of these factors in limiting mangrove carbon turnover. Mangrove forests, long considered detritus-based ecosystems, can only be understood when these belowground carbon cycling processes are captured. Furthermore, data on patterns of belowground carbon in these threatened systems can motivate their conservation, given the value of their ecosystem service of carbon storage, estimated to be worth on the order of 1 billion USD in the Gulf of California’s mangroves alone.