Aggregate formation in chloritic and serpentinitic alpine soils

Variations in soil-water and mineralogical composition of clay particles are important factors governing the formation and stabilization of soil aggregates. We evaluated the effects of these factors on aggregate genesis in two soils with different mineralogical compositions. One developed from chlorite-rich schists and the other from serpentinite, but they have similar organic matter and Fe oxides contents. The <2-mm fractions were submitted to one cycle of wetting and drying, producing two classes of newly formed aggregates (2–5 and 95 mm in diameter). The aggregates differed in water stability, with those from serpentinitic soil being more stable. The amounts of water-dispersible clay (WDC) were lower in the more stable aggregates than in the original <2 mm fraction, and the WDC showed more positive electrophoretic mobility values, indicating its stabilization during aggregation. In the aggregates obtained from chlorite-rich soil, the WDC amounts were similar to those of the <2-mm fractions, and the electrophoretic mobility values were more negative. The aggregation and stabilization processes differed in the two soils and were affected by the clay dispersion/flocculation behavior. The variations in aggregate stabilization were therefore attributed to different clay-clay and clay-sand interactions caused by the mineralogical differences in the clay fractions involved. No differences were found when comparing the two size classes of newly formed aggregates from the same soil. This indicates that the same mechanism produced aggregates of a variety of sizes.