Wetting Rate and Rain Depth Effects on Crust Strength and Micromorphology

Aggregate disintegration and crust formation at soil surfaces exposed to rain are caused by rapid wetting of the dry aggregates and by raindrop impact. The relative importance of these two mechanisms was evaluated by studying the effects of the wetting rate (WR) and raindrop impact on crust strength and crust micromorphology. Two soils, a loess (Typic Haplustalf) and a black soil (Pachic Udic Argiboroll), varying in their organic matter content (15.2 and 42.6 g kg −1 , respectively) and aggregate stabilities, were packed in splash cups, prewetted at WRs of 2 or 50 mm h −1 , and exposed to 5 to 60 mm of simulated rainfall. For the unstable loess soil, crust strength increased and crust microfabric deteriorated with increases in cumulative rainfall but were unaffected by the WR. Conversely, in the black soil with high aggregate stability, changes in crust strength and crust microfabric were affected by the WR; for the slow WR, both phenomena were largely unaffected by increases in cumulative rainfall but, for the faster WR, the phenomena were greatly affected by rain depth and crust formation proceeded rapidly. Both WR and raindrop impacts affect surface aggregate breakdown and crust formation. The energy released by slaking while wetting dry aggregates is, however, considerably higher than that of raindrop impact. Thus, the relative importance of WR and rain depth depends on the stability of the soil aggregates. For soils with stable aggregates, raindrop impact alone is not sufficient to destroy aggregates and form crusts. Rapid wetting of the dry soil is essential for aggregate breakdown and crust formation. For soils with unstable aggregates, the energy of raindrop impact is sufficient for seal formation.