Biological soil crusts, composed of cyanobacteria, lichens, mosses and associated microbes, are a widespread surface layer in drylands that strongly influence soil moisture retention. Research by Ferran Garcia-Pichel at Arizona State University and Jayne Belnap at the U.S. Geological Survey demonstrates that biocrusts are not passive covers: they alter water inputs, losses and storage through multiple interacting processes, with measurable consequences for vegetation, erosion and dust emission.
Mechanisms
Biocrusts affect moisture through changes to evaporation, infiltration and near-surface storage. Surface cover reduces direct soil exposure and can lower evaporation rates by shading and by creating a thin microboundary layer that slows vapor loss, a process emphasized in studies led by Ferran Garcia-Pichel at Arizona State University. Extracellular polymeric substances produced by cyanobacteria increase surface cohesion and can enhance the micro-scale water-holding capacity of the soil surface. At the same time, the physical texture of crusts alters runoff and infiltration: rough, filamentous cyanobacterial crusts tend to increase microtopographic roughness and slow runoff, promoting infiltration, while dense lichen and moss crusts may seal fine pores and reduce infiltration under certain rainfall intensities. Work by Thomas Eldridge at the University of New South Wales documents these variable responses in Australian drylands and highlights that the outcome is context-dependent on crust type, soil texture and storm characteristics.
Consequences and cultural context
The net effect of biocrusts on landscape hydrology cascades to ecological and human dimensions. By modifying moisture availability near the surface, crusts influence seed germination, seedling survival and the distribution of vascular plants. Jayne Belnap at the U.S. Geological Survey has shown that disturbance such as livestock trampling or off-road vehicles that remove crust cover leads to increased runoff, greater erosion and reduced near-surface moisture, amplifying desertification risks. There are also cultural and territorial implications: many pastoral and indigenous communities rely on subtle soil moisture regimes for forage and traditional land uses, so crust loss affects livelihoods. Environmentally, intact crusts reduce dust emissions that can transport nutrients and pathogens regionally, linking small-scale soil processes to broader atmospheric and human health outcomes.
Preserving and restoring biocrusts is therefore integral to maintaining soil moisture retention and the ecosystem services of drylands, but management must account for species composition, disturbance history and local climate to be effective.