DESTABILIZATION OF AGGREGATES IN SOME TYPIC FRAGIUDALFS.

Several mechanisms are responsible of the destabilization of soil aggregates in water: slaking, swelling, and dispersion of the clay and mechanical breakdown by abrasion. The aggregate resistance differs in relation to the dominant disruptive phenomenon, and by applying different methods to assess aggregate stability, a specific physical susceptibility of different horizons may be evidenced. Therefore, we evaluated the relative importance of the mechanisms of breakdown in some Typic Fragiudalfs, taking into account the specific horizon characteristics, to understand if the fragipan brittleness is related to a specific mechanism, and how the soil properties affect the resistance to fast wetting. The standard wet sieving test, which evaluates all destabilization mechanisms together, indicated a clear role of soil organic matter in protecting the aggregates, with greater losses in the deeper horizons (about 70%), but did not allow us to discriminate between fragipans and other horizons. When the losses caused by water abrasion were separated from the breakdown due to wetting phase using a kinetic approach, a high breakdown caused by fast wetting was found in deep horizons, but still no difference between fragipan and non-fragipans were visible, even though fragipan clods are known to be particularly sensitive to fast wetting. By excluding the effect of organic matter and prewetting the samples with ethanol, differences between these horizons appeared within the water saturation phase and fragipans and non-fragipans were found to be sensitive to different mechanisms. In fragipans, the relative percentage of slaking was higher, always above 30%, whereas for non-fragipans clay dispersion and swelling weighted more heavily. The complexity of the clay fraction did not allow us to relate the mineralogy to swelling or dispersion, but slaking was instead clearly related to clay arrangement and the consequent porosity characteristics (r2 = 0.62), and not to clay content only.