Erosion of Amazonian mangroves over peatlands leads to soil carbon loss

Mangrove forests are coastal ecosystems with high soil carbon stocks as a result of intense forest productivity and saturated soils that favor organic preservation for long periods of time. In some regions, the overlap of mangroves with peatlands revealed large soil carbon stocks with great conservation interest. Coastal erosion significantly threatens mangrove forests globally, yet carbon emissions from such losses, especially over peatlands, remain underexplored. Although peatlands are abundant in South America, their overlap with mangroves along the Brazilian Amazon coast has never been fully explored.

We explored mangroves in the Amazon coast of Brazil that were predicted to occur over coastal peatlands, and estimated their soil carbon stocks. We then overlapped the peatland and mangrove distribution on the Amazon coast to quantify their extent, estimated the potential emission factors associated with erosion and loss of their soil carbon stocks.

We discovered shallow organic peat layers beneath Brazilian Amazon mangroves and estimated that a significant portion of Brazil's mangroves overlap with peat deposits. Here we conducted soil sampling and carbon stock analyses on Maracá Island, revealing peat layers 51-71 cm thick with high carbon content. Carbon stock losses from eroded mangrove peatlands were estimated at 100.5 ± 33.6 Mg C ha^-1, representing 72% of soil carbon stocks. These findings highlight the substantial carbon reservoir in Amazonian mangrove peatlands and their vulnerability to erosion-driven carbon release. Recognizing mangroves over peatlands as distinct ecosystems is crucial for accurate carbon accounting and informs conservation strategies aimed at mitigating greenhouse gas emissions from coastal erosion in the Amazon.