Several studies illustrate the wind and water erosion-reducing potential of semi-permanent microbiotic soil crusts in arid and semi-arid desert environments. In contrast, little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter. This study attempts to fill the research gap through (a) a field survey assessing the occurrence of biological soil crusts on loess-derived soils in central Belgium in space and time and (b) laboratory flume (2 m long) experiments simulating concentrated runoff on undisturbed topsoil samples (0.4 x 0.1 m(2)) quantifying the microbiotic crust effect on soil erosion rates. Three stages of microbiotic crust development on cropland soils are distinguished: (1) development of a non-biological surface seal by raindrop impact, (2) colonization of the soil by algae and gradual development of a continuous algal mat and (3) establishment of a well-developed microbiotic crust with moss plants as the dominant life-form. As the silt loam soils in the study area seal quickly after tillage, microbiotic soil crusts are more or less present during a large part of the year under maize, sugar beet and wheat, representing the main cropland area. On average, the early-successional algae-dominated crusts of stage 2 reduce soil detachment rates by 37%, whereas the well-developed moss mat of stage 3 causes an average reduction of 79%. Relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover (b = 0 center dot 024 for moss-dominated and b = 0 center dot 006 for algae-dominated crusts). In addition to ground surface cover by vegetation and crop residues, microbiotic crust occurrence can therefore not be neglected when modelling small-scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments. Copyright (C) 2007 John Wiley & Sons, Ltd.
