MECHANISMS OF AGGREGATE BREAKDOWN INVOLVED IN SURFACE SEALING, RUNOFFGENERATION AND SEDIMENT CONCENTRATION ON LOESS SOILS

The aim of this study was to assess the effect of different mechanisms of aggregate breakdown on sealing susceptibility, runoff generation a nd sediment concentration. Two loess derived soils with similar chemic al and physical properties, but exhibiting differences in sealing susc eptibility in the field, were taken from two seedbeds of contrasting t illage systems and subjected to simulated rainfall. Various parameters to describe aggregation were determined, as well as infiltration, run off and sediment yield. Infiltration rate and total infiltration after a cumulated kinetic rainfall energy of 750 J m(-2) proved to be good indicators of sealing susceptibility. Pronounced differences in both i nfiltration and sediment yield were observed between the soil types an d, in one case, between tillage treatments. No relationship was found between aggregate stability and infiltration parameters. However, when the size distribution of particles produced by aggregate breakdown wa s considered, the variations in sealing susceptibility could be satisf actorily explained. An alternative definition of aggregate stability i s proposed in terms of sealing dynamics, differentiating between rate of aggregate breakdown and size distribution of particles produced by aggregate disruption.