Carbon, calcium carbonate, and nitrogen sources and distribution across mangrove forests: a case study from northeastern Brazil

Mangrove forests are highly efficient sinks of carbon (C) and nutrients. However, the magnitude of C storage and its dynamics across different forests remain poorly understood. Similarly, knowledge gaps persist regarding nitrogen (N) distribution among species-dominated forests. We evaluated the spatial distribution of total C and N along a mangrove forest in Brazil. Soils were sampled from: Laguncularia racemosa , coexistence of species, and Rhizophora mangle forests distributed along the Botafogo estuary. Total C and N in soils and plants, as well as the C/N ratio for plants, were analyzed. The sources of C and N were assessed using δ 13 C and δ 15 N isotopic signatures. Carbonate phases were characterized by SEM-EDS, and total calcium (Ca) concentrations were quantified by XRF. We identified a clear increasing gradient of soil total C from the upper to the lower estuarine zones: L. racemosa forest presented the lowest values (1.34%), whereas R. mangle forest exhibited the highest concentrations (7.44%). Oyster contribution was exclusive to the R. mangle forest, enabling an inorganic C contribution, and a substantial Ca input to the soils, 11 times higher than the three-species coexistence forest, and 8.6 times higher than the L. racemosa forest. Calcium concentrations were positively associated with total soil C, suggesting a potential role of carbonates in organic C stabilization. Overall, the study demonstrates a distinct pattern of C distribution, unveiling the potential of R. mangle forests to release refractory organic C and providing a favorable habitat for bivalve organisms such as Crassostrea rhizophorae . These organisms enhance the input of inorganic C into the system, further contributing to surface soil C storage.