Effects of sand burial on fluxes of greenhouse gases from the soil covered by biocrust in an arid desert region. Response of biocrusts’ photosynthesis to environmental factors:a possible explanation of the spatial distribution of biocrusts in the Hilly Loess Plateau region of China The effects of simulated nitrogen deposition on growth and photosynthetic physiology of three types of biocrusts Surface roughness characteristics of biological soil crusts and its influencing factors in the hilly Loess Plateau region, China