Clay minerals accelerate the formation of indoor biocrusts: Effects and mechanism

The rapid formation or restoration of biocrusts by human intervention, such as constructing artificial biocrusts, is considered a promising technology for desertification control. To date, most of the studies mainly focused on the role of cyanobacteria and their excreted exopolysaccharides (EPSs) in the process of biocrusts formation and development, while few studies have been concerned about clay minerals, which play an important role in soil development as the abiotic factor. In this study, two clay minerals (montmorillonite and kaolinite) were added to crushed natural biocrusts at different ratios(0.5 %, 1 %, 2 %, 5 %, 10 %; w/w), aiming to explore the effects and mechanism of clay minerals on the formation of biocrusts. The results showed that low-level montmorillonite (0.5–2 %; w/w) addition significantly increased the photosynthetic biomass by about 50 % (indicated by Chlorophyll-a and gene copies) and promoted the accumulation of total nitrogen, nitrate nitrogen, ammonia nitrogen, and total organic carbon. The results of the structural equation model showed that the enhancement effect of clay minerals on the formation of biocrusts is mainly ascribed to its direct positive effect on biocrust physical stability and indirect positive effects through increasing both the cyanobacterial biomass and EPS excretion. Notably, the addition of montmorillonite increased the physical stability of the biocrusts by 18 %-53 %, while kaolinite did not play a positive role in the formation of biocrusts. Moreover, excessive montmorillonite (more than 2 %) and kaolinite caused the significant shift of dominance from Cyanobacteria to Proteobacteria, and a sharp decrease in EPS excretion, posing a high risk of biocrust disintegration. Overall, our findings demonstrate the stimulation mechanism of clay addition on restoring natural biocrusts, providing new insight for constructing artificial biocrusts and restoring degraded biocrusts in dryland.