Effects of increased salinity on the destabilization processes of organic matter associated with essential elements in mangrove soil

The understanding of the mechanisms that affect the stability of soil organic matter (SOM) in mangroves, such as the effects of increased salinity, is still limited. Therefore, the aim of this study was to evaluate, through increasing salinity, changes in Fe biogeochemistry, as well as in SOM associated with Al, Fe, Ca, and Mg. The experiment was conducted using mangrove soil collected in Cananéia, São Paulo, Brazil, comprising 24 sampling units divided into four treatments with different salinity levels (28, 35, 42, and 50 PSU), each with six replicates, conducted for 90 days with simulated tidal oscillation. Sequential wet chemical extractions were used to obtain information on organo-mineral associations, using different extractants (sodium citrate, citrate–ascorbate, and dithionite–citrate–bicarbonate). Element concentrations in the extracts were determined using ICP-OES. The main results indicated that increased salinity caused significant losses in Mg concentrations associated with SOM (19%) in the citrate–ascorbate extractant and in Fe concentrations (33%) in the citrate extractant. when comparting the lowest (28 PSU) with the highest salinity treatment (50 PSU). These results may indicate a disturbance in organo-mineral complexes due to increased competition with ions from the solution. Thus, increasing salinity in mangrove soils may destabilize organic matter and undermine their role as carbon sinks.