Biocrust islands enhance infiltration, and reduce runoff and sediment yield on a heavily salinized dryland soil

•Lichen and moss-dominated biocrusts reduced runoff and sediment loss.•Biocrusts were associated with non-saline soils of greater sand content.•Biocrusted soils had greater cumulative infiltration and hydraulic conductivity.•Biocrusts act as infiltration islands that could potentially regenerate saline soil. Salinity and sodicity are major forms of land degradation in drylands worldwide, reducing soil function, and threatening pastoral livelihoods. Dryland soils are often dominated by biocrusts, surface aggregations of lichens, bryophytes, fungi and other minute organisms that stabilize surface soils. Biocrusts could have a role in maintaining hydrological functions in heavily salinized areas, but there have been few studies, and the mechanisms are poorly understood. We tested whether biocrust mosses and lichens enhanced infiltration, and reduced runoff and sediment yield, on biocrusted islands scattered among extensive patches of bare highly saline soils in northeaster Iran. Biocrusted soils had greater cumulative infiltration and hydraulic conductivity, less runoff, which commenced later, and lower sediment yields than bare soils. The water content of biocrusted soils was greater than bare soils, but only at low matric potentials. Biocrusted surfaces were alkaline, more sandy, had lower levels of sodium, chloride, and calcium-plus-magnesium ions, and a lower sodium absorption ratio than bare soils. Structural equation modelling showed that increasing salinity was associated with a suppression of the negative effect of biocrusts on runoff. Potential mechanisms for reduced runoff likely relate to enhanced soil aggregation and porosity of biocrusted soils leading to greater retention of soil water content at low matric potentials, as well as an ability of mosses and lichens to capture and store surface water or to permit infiltration into the uppermost surface layers. Overall, our findings reveal a strong association between moss- and lichen-dominant biocrusts and soil hydrological processes, and suggest that these crusts play an important role in maintaining hydrological function in heavily salinized soils.