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Are mangrove forest substrates sediments or soils? A case study in southeastern Brazil

Abstract:

Morphological and analytical data corresponding to several profiles from two mangrove systems in the Brazilian state of São Paulo were examined for evidence of pedogenetic processes. Plant activity exerted a strong effect on the processes occurring in mangrove substrates, especially on the intensity of these processes and, therefore, constitutes one of the major factors involved in the differentiation of sediments and soils. Colonization of substrates by vascular plants leads to drastic changes in physicochemical conditions. The redox processes were much more diverse in the soil than in the sediment (substrate without vegetation); in the former they varied with depth, with oxic or suboxic conditions being observed at the surface and anoxic conditions observed below 30 cm, whereas in the sediment, anoxic conditions were observed throughout. Likewise, the acid-base conditions were more variable in the soils, ranging from strongly acid to neutral, and in the sediment they were close to neutral. Furthermore, different pedogenetic processes were identified: (1) addition of organic matter by accumulation of vegetable debris and dead roots, with formation - in some cases - of a histic epipedon, (2) transfer of soluble iron (Fe2+) towards the surface and precipitation in the form of Fe oxyhydroxide, and transfer of the mineral particles due to the bioturbation caused by activity of crabs, (3) transformation of elements such as iron (gleization) and sulphur (sulphidization). Changes undergone in the mangrove substrates were also characterized by simultaneity and intergradations between pedogenesis and diagenesis, especially in the lowermost layers in which authigenesis appears to intensify. Thus, the presence of smectite minerals in the clay fraction may be attributed to authigenic processes that take place in both soils and sediments. © 2006 Elsevier B.V. All rights reserved.