Wetlands can store large quantities of carbon (C) and are considered key sites for C sequestration. However, the C sequestration potential of wetlands is spatially and temporally variable, and depends on processes associated with C production, preservation and export. In this study, we assess the soil C sources and processes responsible for C sequestration of riverine wetlands (mangroves, peat swamp forest and marsh) of La Encrucijada Biosphere Reserve (LEBR, Mexican south Pacific coast). We analysed soil C and nitrogen (N) concentrations and isotopes (d13C and d15N) from cores dated from the last century. We compared a range of mangrove forests in different geomorphological settings (upriver and downriver) and across a gradient from fringe to interior forests. Sources and processes related to C storage differ greatly among riverine wetlands of the Reserve. In the peat swamp forest and marsh, the soil C experienced large changes in the past century, probably due to soil decomposition, changes in plant community composition, and/or changes in C sources. In the mangroves, the dominant process for C accumulation was the burial of in situ production. The C buried in mangroves has changed little in the past 100 years, suggesting that production has been fairly constant and/or that decomposition rates in the soil are slow. Mangrove forests of LEBR, regardless of geomorphological setting, can preserve very uniform soil N and C for a century or more, consistent with efficient C storage.