Flooding effect on mineralogical and geochemical changes in alkaline-sodic soil system of northern Pantanal wetlands, Brazil
Résumé
In the northern Pantanal wetlands of Brazil, alkaline sodic soils play a significant role in the ecological functioning of the region, but little is known about their mineralogical equilibria in current flooding dynamics which is necessary for good management of these areas. This study focused on a transition between sodic and non-sodic soils, identified by low frequency electromagnetic induction survey. Samples of B horizons from alkaline-sodic and non-sodic soils were analyzed via soil solution from saturated paste, decomposition of X-ray diffraction (XRD) patterns and Transmission Electron Microscopy with X-ray dispersive energy spectroscopy (TEM-EDS) analysis. The transition between alkaline-sodic and non-sodic soils matches with the transition between flooded and non-flooded areas. The soil solution shows a concentration factor of about 40 along this transition. The formation of sodic soils results from an accumulation by evaporation of solutions with a positive calcite residual alkalinity (RAcalcite), probably imposed by the chemistry of the São Lourenço River. An increasing Na:Ca ratio caused solonization of the exchange complex. These alkaline-sodic soils, however, can be interpreted as declining soils in the current context with an evacuation of soluble salts during flood cycles. The detailed investigation of the fine clay fraction (< 0.2µm) using full profile modelling and decomposition of XRD patterns and TEM-EDS analysis demonstrates gradual transformations of trioctahedral to dioctahedral smectite by Mg-Al substitutions on octahedral sites, leading to the prevalence of kaolinite-smectite interstratified minerals when the conditions become more acidic via solodization process.
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