Runoff generation fostered by physical and biological crusts in semi-arid sandy soils

Biological soil crusts occur extensively in semi-arid regions. In the western part of Niger, they are associated with various types of physical soil crusts in fallows and in the "tiger bush ecosystem" (a landscape with a typical banded pattern consisting of densely vegetated bands of small trees and shrubs alternating with bare soil bands). Despite their widespread occurrence, not much research has been done regarding their role on runoff generation in the Sahelian environment. The present study aims at: (i) highlighting the relative contribution of biological soil crusts in infiltration and runoff on crusted soil in respect with other soil factors; (ii) assessing the impact of underlying physical crusts associated with a wide range of microbial cover; and (iii) gathering the data required for an evaluation of the potential of microbial cover in water redistribution in an arid environment and for modelling purposes. The study was based on runoff measurements performed in situ at the level of ten plots, 1 m(2) each, under two simulated rain-falls. Those experiments were coupled with the visual characterization and image analysis of soil surface conditions as well as micromorphological examinations, using a light microscope, both on the field and at the laboratory. Visual estimation of crust cover was found consistent with the estimation based on the image analysis. Biological soil crusts capped the summit of pre-existing physical soil crusts, i.e. structural crusts, formed in situ by the impact of water drop and erosion crusts, resulting from water or wind erosion of structural crusts. The percentage of microbial cover in the 1-m(2) plots range from 39 to 80% (55% in average) on structural crusts compared to 4 to 29% (17% in average) on erosion crusts. Runoff coefficient varied with both the level of microbial cover and the type of physical soil crust. Higher runoff coefficients were found where biological soil crusts had developed on erosion crusts as compared to structural crusts. Runoff coefficients recorded from erosion crusts increased with the level of microbial cover while negative correlations were found on structural crusts. They show that the clogging effect of microbial cover for the plasmic layer of erosion crusts is higher than that of the sand layer of structural crusts. Separating biological soil crusts according to the types of the underlying physical crusts explain, therefore, the contradictory findings related to their role on infiltration and runoff generation. Those data will help to evaluate the potential role of biological soil crusts in the modelling of water redistribution in the arid environment of the western part of Niger