Effect of biocrusts on profile distribution of soil water content and salinity at different stages of evaporation

In this study, the effect of biocrusts on daily variations of evaporation in soils of an arid rangeland was investigated during 142 days. Temporal and spatial variations of soil water and solute content were analyzed in three times of 17, 50 and 103 days, and four depths of 10, 20, 30 cm and 40 cm in six columns (20 cm diameter × 50 cm height) containing biocrusts (43 cm saline soil + 5 cm biocrusts) and four columns filled with bare soil (48 cm saline soil) respectively. Temporal variation of evaporation rate was supposed as a 3-stages process. Results showed that stage 1 and stage 2 are not distinguishable. We further found that, compared to bare soil, the biocrusts retarded water evaporation from soil surface (24 vs. 30 mm and 34 vs. 44 mm after 17 and 50 days respectively), and reduced solute accumulation in top soil, especially in the stage 2 of evaporation process (5 vs. 8 dS/m and 13 vs. 20 dS/m after 17 and 50 days respectively). The effect of biocrusts on the evaporation rate at the stage 3 of evaporation was not significant (9 vs. 9 mm after 103 days). Reduction of evaporation by biocrusts resulted in less soil salinity within entire soil profile. The effects of biocrusts on evaporation reduction are attributed to i) the biocrusts create mats on the soil surface, and swelling of mats blocks the soil pores in wet condition, and ii) the large pores of biocrusts disrupt the hydraulic connectivity between topsoil and subsoil, which minimizes the capillary rise. Since major part of water is evaporated during stage 2, and is controlled by water flow within the soil and at the surface, so conservation of the BSCs would be important for preserving hydrological functions advantage such as reducing evaporation and enhancing water retention in the salt affected and dry soils.