Origin of microaggregates in soils with ferralic horizons

Microaggregates that characterize ferralic soils have been hypothesized to have physical, geochemical and/or biological origins. Despite of many studies, the hierarchy between these processes that form microaggregates has seldom been reported. The objective of this work was to study the genesis of microaggregates in a sequence of Ferralic Nitisols developed on Quaternary red clayey sediments and diabase in Piracicaba (SP), Brazil. This issue was tackled by combining optical microscopy, image analysis, scanning electron microscopy and elemental iron quantifications by X-ray fluorescence. Micromorphological investigations showed three different types of microaggregates: (i) oval microaggregates with well sorted quartz grains in their interior; (ii) oval microaggregates without or with poorly sorted quartz grains in their interior; and (iii) dense polyedric microaggregates. These morphological evidences, together with the elemental iron determinations and scanning electron microscopy, revealed the contribution of more than one process for microaggregate formation: (i) the mechanical action of the mesofauna would form the first type of microaggregates (ii) geochemical and biological processes would form the second type and (iii) the fissuration of the soil matrix by expansion and compression processes would form the third type.