Acid neutralization and sulphur retention in s-impacted andosols
(2005)
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- (en) While Andosols have a proven capacity to buffer acid inputs, their long-term chemical response to elevated acid deposition remains poorly known. In this respect, the high anion retention capacity of Andosols constitutes a key parameter. Yet, the mechanisms involved in anion retention, especially sulphate, are still a matter of scientific debate. In this study, we report on the impacts of volcanogenic S and acid depositions on (i) the sulphate distribution and (ii) the processes involved in the neutralisation of the acid inputs, in two distinct soil series located downwind from Masaya volcano (Nicaragua), one of the world's largest natural source of SO2. The first series corresponds to weathered Eutric Andosols rich in allophanic constituents and the second series to weakly developed Vitric Andosols rich in volcanic glass. Long-term acid gas emission by Masaya volcano has led to important changes in the chemistry of the Andosols downwind. Sustained acid inputs have decreased the pH and exchangeable base cations contents in both Vitric and Eutric soils. These soils also show substantial S enrichment (up to 5470 mg S kg-1). However, these changes do not affect the soil acid neutralising capacity of the solid phase (ANCs) in a significant way. Despite the larger ANCs of the Vitric comparatively to the Eutric Andosols, soil pH was less in the Vitric than in the Eutric Andosols. This is related to the naturally and kinetically different mechanisms involved in the regulation of the volcanogenic acid fluxes: mineral weathering (slow kinetics) is the dominant process in Vitric Andosols, whereas cation exchange and sulphate sorption (rapid kinetics) significantly contribute to regulate proton consumption in Eutric Andosols. Formation of basic aluminium sulphate (BAS) [(K,Na)nAlx(OH)y(SO4)z] minerals in soils exposed to volcanogenic S-rich acid inputs was inferred from the results of selective extraction experiments (NH4F, KH2PO4 and oxalate). Precipitation of BAS probably constitutes the most effective inorganic SO42- retention mechanism (9-51% of total S), SO42- adsorption onto soil constituents (1-36% of total S) and occlusion into short-range ordered minerals (0-22% of total S), probably governed by ferrihydrite, constituted additional effective inorganic SO42- retention processes. Using transmission electron microscopy coupled with energy-dispersive analysis, we provide the first direct evidence of BAS minerals in soils. BAS minerals can form in these Andosols, because of the large inputs of H+ and SO2 and the availability of readily weatherable volcanic glass, which acts as an effective source of Al. Surface precipitation, i.e. two-dimensional chemisorption followed by three-dimensional nucleation and precipitation, rather than direct precipitation from solution, is likely the formation pathway of BAS particles in these soils.
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Citations
Delfosse, T. (2005). Acid neutralization and sulphur retention in s-impacted andosols. https://hdl.handle.net/2078.5/80633