Clay minerals: Precise markers of the spatial and temporal variability of the biogeochemical soil environment

As clay minerals develop a large surface area, they have a considerable capacity to react with the other soil phases and eventually record the subtle chemical changes (in space and time) occurring during these interactions. To test this hypothesis, we characterized the spatial variability of forest soil properties (at the infra-millimetre scale) around the roots. Temporal variations of these properties were followed over a period of 3 months. Chemical extractions and mineralogical studies indicated that the bulk soil and the rhizospheric soil fraction presented different characteristics which changed with the seasons. In agreement with Si and Al extracted by the citrate treatment (Sic, Alc), XRD behaviour of the 2:1 phyllosilicates reflected temporal and spatial variations of the interlayer occupation. In addition, both the dissolution and precipitation of minerals were observed. In the bulk soil, weathering was more pronounced in June and resulted in an increase in the amount of Sic in the surface layer and Alc amounts in layers 2 and 3. The increase in the amount of Alc was attributed to aluminium compounds precipitated between March and June, mainly located outside the clay interlayers and only slightly polymerised. The high weathering rate in June was attributed to microbiological activity, and especially to organic matter mineralisation associated with nitrification and proton production. In the rhizosphere, uptake of nitrates in June resulted in the excretion of large quantities of OH− around the roots. Higher hydroxylation of the aluminium compounds in these sites was shown by a reduction in the aluminium-hydroxide charge, weaker collapse of the clay interlayers and a decrease in the CEC. Also, the excess of K input by mass flow and mineral weathering relative to root uptake led to an increase in K in the rhizosphere solution, fixation of K in the interlayers and irreversible collapse of part of the expanding minerals. This process should have a decelerating effect on weathering and aluminisation of the vermiculites. The unleached part of K remains available for plant nutrition when the environmental conditions change.