Grazing disturbance reduces biocrust-related cyanobacteria and N-fixer abundance but increases bacterial diversity: Implications for biocrust restoration in degraded drylands

Overgrazing is a major driver of dryland degradation, however, so far, there is limited understanding on how this process affects biocrust-related microbial community, and especially how the key groups respond to grazing disturbance. In this study, quantitative polymerase chain reaction (qPCR) and high throughout sequencing technologies were used to investigate the bacterial community abundance and diversity in the Horqin Sandland (China) experiencing different livestock grazing disturbances, in order to examine whether a shift in bacterial community (in particular the key biocrust components, cyanobacteria) was involved, and how this was related to biocrust development and altered soil carbon and nitrogen level. Our results revealed that a clear heterogeneous soil bacterial community was associated with grazing disturbance, which inhibited biocrust development. The decreased photosynthetic cyanobacterial abundance (81.81 % in relative abundance and 98.83 % in absolute abundance) and nitrogen-fixing genes (87.72 %), associated with the lower total carbon and nitrogen content (P<0.05), illustrated a low soil carbon and nitrogen-fixing capability and nutrient level in the grazing-disturbed soils. In particular, a switch of nitrogen-fixing dominance from cyanobacteria to proteobacteria was induced by the grazing disturbance. Compared to the grazing-disturbed soils, higher dominant cyanobacterial operational taxonomic units (OTUs) (especially the species of Nostoc and Scytonema) and lower bacterial diversity (e.g., Shannon, Ace, and Chao index) were observed in the undisturbed biocrust soils, highlighting that the dominants rather than diversity played more important roles in biocrust development. Collectively, our results demonstrate that cyanobacteria are very sensitive (even more than others, e.g., proteobacteria and actinobacteria) to grazing disturbance, therefore, we propose that cyanobacterial inoculation is likely an effective approach to supplement soil cyanobacterial abundance, which is expected to induce biocrust development and increase carbon and nitrogen level in the disturbed drylands.