Partitioning autochthonous and allochthonous carbon in mangroves of the Brazilian Amazon coast

• Strong spatial differences in mangrove organic sources due to the Amazon River. • Autochthonous carbon predominates (>54%) in estuarine mangroves. • Allochthonous carbon makes up 34–38% of surface soil organic carbon. • Mangroves in deltaic settings have a 30% lower restoration value. Mangrove forests are important coastal carbon sinks, yet the origins of their soil organic carbon can vary among different coastal geomorphic settings. Understanding the relative contributions of autochthonous and allochthonous organic matter in mangroves of the Amazon coast adds important ecological information on the processes driving carbon accumulation and can help guide future restoration efforts in Brazil. Here we applied stable carbon and nitrogen isotope analysis combined with Bayesian mixing models to partition organic matter sources from 496 soil samples in deltaic and estuarine mangrove of the Amazon coast. Autochthonous sources dominated soil organic carbon in deltaic (54 %) and estuarine (67 %) mangroves. The Amazon River contributed to 24 % of the allochthonous inputs to deltaic mangroves (3-fold larger than estuarine settings), highlighting their connection to the health of the freshwater basin. Our models suggest that mangrove carbon contributes substantially the surface soil layer (0−50 cm, >50 % of autochthonous carbon), and suggests a higher value of restoring mangroves in estuarine settings for increased blue carbon credit benefits. This work supports the role of mangroves of the Amazon coast in climate mitigation strategies and guides the valuation of potential restoration projects with region-specific data.