Biological soil crusts for restoring arid soils: functions, inoculation, and field scaling

Arid and semi-arid regions occupy a substantial proportion of the global land surface and are highly susceptible to soil degradation driven by climatic stress and human disturbance. Biological soil crusts, composed of cyanobacteria, algae, lichens, mosses, and associated microorganisms, are fundamental regulators of soil stability, hydrological processes, and biogeochemical cycling in these ecosystems. This review synthesizes current understanding of the composition, development, and ecological functions of biological soil crusts, with a particular focus on their role in dryland restoration. We examine how crust structure and successional stage control key ecosystem processes, including erosion resistance, water redistribution, carbon sequestration, and nitrogen transformations, while also addressing emerging controversies related to functional redundancy and reactive nitrogen emissions. The review further evaluates restoration strategies based on biological soil crust inoculation, comparing field-sourced and laboratory-cultured approaches, and assessing how inoculum selection, application techniques, and habitat amelioration influence establishment success. Special attention is given to the challenges of translating plot-scale successes to field-scale applications, where environmental heterogeneity, disturbance regimes, and economic constraints strongly affect outcomes. By integrating evidence from long-term field studies, experimental manipulations, and recent meta-analyses, this review highlights the context-dependent nature of biological soil crust restoration and emphasizes the need for site-specific, function-oriented assessment frameworks. The synthesis provides guidance for researchers, land managers, and policymakers seeking to design effective and sustainable restoration strategies for degraded dryland soils.