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Pedogenetic processes in mangrove soils

Abstract:

Despite the acknowledged environmental, economic and social importance of mangroves, one important question remains: should mangrove substrate be considered soil or sediment? This chapter presents results obtained from several studies performed with mangrove soils from the State of São Paulo (Brazil) and based on the four soil forming processes: additions, losses, transformations and translocations. These results provide evidence for the occurrence of soil processes that lead to the transformation of deposited sediments into soil. Such changes are induced by the transformation of organic matter in anoxic and suboxic environments, combined with the action of vegetation and bioturbation by macrofauna. After a brief discussion of sedimentological and mineralogical aspects associated with the formation of these environments during the Late Quaternary, some physicochemical data are presented. The results show a greater degree of variability in physicochemical parameters in soils (vegetated substrates) than in non-vegetated substrates. Inputs of organic matter are tend to be higher in vegetated substrates, primarily due to the root biomass, which leads to an oxidizing environment and to higher concentrations of reactive forms of Fe (Fe oxides and poorly-crystalline compounds), especially in surface layers. This process is related to oxidization of iron sulfides and to a decrease in pyrite-Fe. The results also indicate a gradual increase in the sulfidization processes with depth, due to decreases in the amount of roots and bioturbation by macrofauna. The importance of biotic factors in controlling the four pedogenic processes on mangrove substrates is therefore confirmed. Further evidence of pedogenesis is also provided by the increasing degree of crystallinity of the clay minerals (mainly smectites) with depth, indicating a gradual dominance of diagenesis over pedogenesis in deeper layers. Finally, aspects of soil classification are discussed and related to the physicochemical processes observed in mangrove soils.© 2011 Nova Science Publishers, Inc. All rights reserved.