Combination of Chemical and Cyanobacterial Inoculation Promotes Biocrust Development: A Novel Perspective for Combating Desertification

Drylands cover a substantial proportion of Earth’s terrestrial surface, but dryland vegetation contributes only a very limited proportion of the terrestrial biomass (less than 1%). (1) Conversely, approximately 30% of the dryland surfaces are covered by biological soil crusts (biocrusts). (2) Biocrusts are associations of phototrophic and heterotrophic communities, which dwell within the uppermost millimeters of soil surfaces. (2,3) They are ecosystem engineers involved in surface stabilization and nutrient cycling, and therefore are regarded as important agents for combating desertification. (3) In a recent published article on ACS Sustainable Chemistry & Engineering, Chi et al. proposed a new idea to promote biocrust development and increase soil nutrient levels by inoculating cyanobacteria and nanosand-stabilizers to dryland surfaces. (4) They corroborated the feasibility of this approach by a 10-month field experiment (1 m × 1 m). As a premise of large-scale engineering applications, this work provides a novel basis for combating desertification and points out the new potential of cyanobacterial inoculation technology to promote biocrust formation and dryland ecological restoration. Artificial induction of biocrust development may be managed in a variety of ways. Since the last century, several technologies have been gradually proposed. (5−7) One example is the dispersion of crushed biocrust material, dried or as a slurry. (5) However, due to the limited amount of producible inoculum, treatments over large scale using such approaches are challenging. Conversely, cyanobacteria are easy to cultivate and harvest to achieve large amount of biomass, (6,7) which is required for large-scale biocrust engineering. In the research of Chi et al., they even introduced water-bloom (aquatic) cyanobacteria to promote biocrust development, an approach which has been proved highly effective for dryland ecological restoration, as well as for alleviating water eutrophication. (4) Furthermore, large-scale inoculation with biocrust-dwelling cyanobacteria Microcoleus vaginatus and Scytonema javanicum has been carried out over more than 40 km2 in Inner Mongolia, China, to induce biocrust development for combating desertification. (6) Therefore, on the basis of the researches of Chi et al. (4) and others, (3,6,7) artificially induced biocrusts through cyanobacterial inoculation technology may be one of the most promising approaches for large-scale and sustainable restoration of degraded dryland soils. Cyanobacterial inoculation can also be optimized further depending on the inoculation settings, and its probability of success increased by adding selected fixing chemicals to the final inoculum formulation, which can support the ability of cyanobacteria to increase soil aggregate stability. (4) Although the addition of soil-fixing chemicals for biocrust development has already been proposed, (8) comprehensive studies of the economic and ecological impact of these adjuvants are very limited. Noteworthy, in one field study the application of chemicals sorted out less effective than common mechanical fixing methods (e.g., straw checkerboards). (9) However, the combination of chemical and cyanobacterial inoculation by Chi et al. represented a step-forward. They not only successfully used a network-structured nanocomposite consisting of sodium polyacrylate, xanthan gum, and attapulgite to assist the fast formation of cyanobacteria-induced biocrusts, but more importantly, the nanocomposite was proven biosafe. (4) Therefore, the study proved that the employment of selected chemical agents may represent a new developmental direction for cyanobacteria-based biocrust induction technology. In this regard, selecting the proper chemical assistant agents depending on the inoculated cyanobacterial species and inoculation settings is an important starting point for developing this technology. In the future, more chemical assistant agents should be identified and tested, with looking at a broad range of application areas, geographic regions, and experimental time scales.