Shrub canopy effects on moss biocrust, soil properties, and microbiomes in a semi-arid ecosystem: Implications for ecosystem function and sustainability

Shrubs play a pivotal role in shaping the growth and dynamics of biological soil crusts (biocrusts) in arid and semi-arid regions through various biotic and abiotic mechanisms. Biocrusts, in turn, significantly influence soil microbiomes. However, the precise effects of shrubs on soil microbial communities within biocrusts remain poorly understood. Here, we investigated the bacterial and fungal communities within moss biocrusts situated beneath and between two predominant shrub species (Artemisia ordosica and Salix psammophila) in the Mu Us Sandland, China, using high-throughput sequencing and random matrix theory (RMT) through a network-based approach. Our findings revealed that shrub species and their canopy affect moss biocrust growth, soil properties, and microbial communities. Shrub species significantly affected moss biocrust thickness, soil water content (SWC), total phosphorus (TP), fungal community composition (e.g., Ascomycota) and structure, and microbial network structure. Moss biocrusts beneath and between shrubs differed in coverage, SWC, soil nutrients (e.g., TP, soil organic carbon (SOC), and total nitrogen (TN)), and microbial community composition (e.g., Actinobacteria), structure, and network attributes. Structural equation modeling (SEM) further validated that the canopy-induced changes in microbial community composition primarily stemmed from negative effects on the soil nutrient index (path coefficient = −0.709) and positive effects on SWC (path coefficient = 0.996). Overall, our findings suggest that shrub encroachment initiates a feedback loop favoring shrub establishment and growth at the expense of biocrusts, potentially reducing the capacity of biocrust ecosystems to sequester carbon, with far-reaching implications for ecosystem functionality and sustainability in water-limited regions. While shrub canopy strongly affects microbial community composition within biocrusts, its indirect effects are mediated predominantly through changes in soil properties, offering valuable insights into the intricate associations between soil microbes and shrublands within semi-arid ecosystems.