Trampling of microphytic crusts on calcareous soils, and its impact on erosion under rain-impacted flow

Twenty five intact soil monoliths dominated by microphytic crusts were tested for their susceptibility to erosion by raindrop-impacted flow transport after subjecting them to simulated sheep trampling. Using a sheepʹs hoof, five levels of trampling representing stocking rates ranging from nil to 1.6 sheep ha−1, were imposed on the monoliths. An erosive stress was applied to each surface using a simulated rainfall intensity of 65 mm h−1 (2.7 mm raindrops), surface flow depths of 4 mm and 8 mm with a flow velocity of 25 mm s−1. There was a significant effect of trampling on erodibility (P=0.003), and trampling explained 33% of the variability in erodibility. Only the severe trampling treatment had significantly higher erodibility compared with the other treatments. Compared to the control, severe trampling resulted in a significant decline in the area of the surface covered by micro-depressions and attached microphytic crust, and concomitant increases in loose, detached microphytic crusts, coarse lag gravel and bare soil. The results were supported by micromorphological examination which revealed that trampling and subsequent rainfall simulation resulted in considerable surface sealing and sorting of sediments, loss of fine material and deposition of coarse material in micro-depressions.