Several factors can affect the behavior of soil structure. Amongst them, the organic carbon (OC) content and the content and type of iron oxides present in the soil have been related as the most important. This work is aimed at evaluating soil aggregate behavior regarding to the role of organic carbon, and forms of iron oxides in two soils developed from basalt in subtropical Brazil. Aggregates with size less than 8 mm of an Udorthents and a Paleudult under forest, pasture and conventional tillage of the steep backslope of Sul Riograndense plateau were shaked in water during 0, 30, 60 and 90 minutes. The aggregate distribution in three size classes was measured as well as the content of OC, iron extractable with ditionite-citrate-bicarbonate (Fe d) and ammonium oxalate (Fe o) in the aggregates. The Udorthents showed greater content of soil OC than Paleodult and the forest soil showed greater content of soil OC than pasture and conventional tillage. The content of Fe d and Fe o were similar between both soils and uses. The studied soils showed a differentiated degree of aggregate hierarchy (AH). The most stable aggregates (90min of shaking) presented a trend to possess greater content of OC than the less stable ones (0min of shaking). Amongst the variables considered in this study, the content of OC was the attribute that more affects the expression of AH and the action of iron oxides in the aggregation not only depends on its type and amount, but also of the content of soil OC. It was also observed that the most stable aggregates (90min of shaking) tended to present greater content of Fe d and smaller content of Fe o.
stabilizing agents; aggregate hierarchy; soil structure
