The effects of microbiotic soil crusts on soil water loss

Microbiotic soil crusts play several important roles in many arid and semiarid ecosystems around the world. Their effects on soil hydrology, however, are poorly understood. It has been speculated that crusts ( 1) improve soil water availability by sealing the soil surface to water loss, or ( 2) diminish soil water by increasing the latent heat of a soil pro le thereby increasing evaporation. In order to distinguish between these two opposing hypotheses, we analyzed water loss and temperature of soil profiles covered by different types of microbiotic soil crust ( cyanobacteria, Psora decipiens, Placidium squamulosum) and a bare soil. We conducted experiments under growth chamber and field conditions. After watering to field capacity, surfaces (1-3 cm deep) of crusted soils took as long or longer to decrease to -1.5 MPa water potential as bare soil in both the growth chamber and the field. In the growth chamber, soils covered by each of the crust types took longer to dry than bare soil. In the field, soils covered by cyanobacteria took longer to dry than bare soils or those covered by the other crusts. Daily soil temperature was not different among the different crust types and bare soil in either experiment. This study did not support the hypothesis that crusts dry a soil pro le more quickly by increasing the amount of energy available for evaporation. Rather our data suggested that crusts increase soil water availability by increasing water flux resistance, specifically at the soil surface.