Influence of Geomorphological Setting and Sedimentary Environment on Soil Organic Matter Content in Neotropical Mangroves

Geomorphological setting and sedimentary environment are key controls on mangrove soil development and organic matter dynamics. This study quantified and compared soil organic matter (SOM) across mangroves established in two contrasting sedimentary environments: an open-coast carbonate system with minimal riverine influence and a deltaic terrigenous system characterized by fluvial inputs. We tested the hypothesis that higher allochthonous inputs in terrigenous environments influence SOM accumulation. Two stations were established within each environment, and soil profiles were sampled to 1 m depth to determine SOM (%) and associated physicochemical properties. SOM varied markedly across environments, with higher values in the carbonate environment (55.2–78.6%) compared to the terrigenous environment (3.6–34.7%). In contrast, porewater temperature (28.4–30.0 °C) and pH (6.8–7.1) showed limited variation among sites. The main gradients were associated with bulk density (0.13–0.94 g cm⁻³), salinity (8.4–38.2), and soil depth (0.92–3.7 m). Principal component analysis indicated a tendency for the carbonate environment to be associated with higher salinity and SOM density; although there was considerable overlap with the terrigenous environment. The high SOM values in the carbonate environment, where allochthonous inputs were minimal, indicate a significant contribution of autochthonous production to soil formation. This finding suggests environmental controls on SOM dynamics in Neotropical mangroves that must be considered to improve assessments of carbon storage across coastal settings.