Carbon sequestration in semi-arid mangrove soils of northeast Brazil

This is the authors' abstract. We don't add key points for this paper.

Mangrove soils can act as long-term carbon sinks, yet carbon sequestration rates remain poorly quantified in many semi-arid and urban-influenced regions, including northeast Brazil. This study estimated soil carbon sequestration rates in upper and lower fringing mangroves across three estuarine systems in the Fortaleza metropolitan region: Rio Ceará, Rio Cocó and Rio Pacoti. Sediment cores were analysed for organic matter, particle size, dry bulk density and ²¹⁰Pb-derived geochronologies to estimate sediment accretion and carbon sequestration over recent decades. Mean carbon sequestration rates varied strongly among estuaries and mangrove positions, with values of 220 and 202 g m⁻² yr⁻¹ in Rio Ceará lower and upper mangroves, 3248 and 592 g m⁻² yr⁻¹ in Rio Cocó lower and upper mangroves, and 890 and 1553 g m⁻² yr⁻¹ in Rio Pacoti lower and upper mangroves, respectively. The mean rate recorded in the Rio Cocó lower mangrove was approximately 14 times higher than the global mean estimate of 226 g m⁻² yr⁻¹ reported by Hopkinson et al. (2012). Elevated rates in Rio Cocó and Rio Pacoti are consistent with external organic matter inputs, including urban-derived material, catchment and aquaculture inputs, and aeolian material from adjacent dune and Caatinga systems, although direct source apportionment was not undertaken. These findings highlight the importance of semi-arid Brazilian mangroves within blue-carbon assessments and indicate that effective conservation should consider mangrove extent, sediment processes, water quality, hydrological connectivity and pressures from urban and coastal development.