Biological soil crusts (BSCs), comprising 12% of the terrestrial surface area and 70% of the dryland surface soil (Rodriguez-Caballero et al. 2018), are a biological complex of microbiomes, cryptogams (algae, lichens, and mosses), and secretions (Velasco et al. 2017). BSCs are considered as ecosystem engineers, particularly in dry land areas (Bowker et al. 2005), because of their critical role in controlling carbon gain (Kidron et al. 2015) and nitrogen cycling (Elbert et al. 2012; Suet al. 2012), regulating soil hydrology (Strausset et al. 2012), and promoting soil nutrients (Chamizo et al. 2012). Soil quality and microenvironment are significantly improved with the development of crusts; therefore, BSCs have great potential in ecological restoration and desertification control (Zhang et al. 2021). Microorganisms are fundamental components of the soil biocrust and are the main performers of their ecological functions (Housman et al. 2006; Ma et al. 2016). Cyanobacteria and proteobacteria are the dominant bacteria involved in carbon accumulation and nitrogen cycle (Liu et al. 2020). Microbial communities in biocrusts are extremely sensitive to environmental changes (Abed et al. 2019; Nuttapon et al. 2020), as well as natural or anthropogenic disturbances (Steven et al. 2015; Velasco et al. 2019). Moreover, climate change is also an essential driver of microbial communities in biocrusts (Blay et al. 2017). As an important component of the terrestrial ecosystem, studying microbial community variation with biocrust succession and their responses to climate changes will enhance our understanding of the effects of climate change on structures and functions of terrestrial ecosystems (Delgado-Baquerizo et al. 2016; Wang et al. 2017).
Vital activities and geochemical processes are largely determined by water and heat availability, and the same is true for microbial communities inhabiting the BSCs. Global climate changes, characterized by warming and changes in precipitation patterns, significantly affect microbial community composition and functions in BSCs (Johnson et al. 2012; Ferrenberg et al. 2015; Hagemann et al. 2015; Bowker et al. 2016; Reed et al. 2016). For example, cyanobacteria in BSCs are considered bioindicators of climate change because of their different responses to warming and precipitation decrease (Muñoz-Martín et al. 2019). Various types of BSCs, such as algae, lichen, and moss biocrusts, represent different succession stages of biocrust. Algal biocrusts, the early stage of biocrusts, create conditions favorable for the colonization of moss and lichens by improving soil quality (Song et al. 2014). Microbial community changes with biocrust development (Zhang et al. 2016; Wang et al. 2020). Moreover, various biocrust types respond characteristically to climate change. For example, climate warming reduces the cover, richness, and evenness of lichen biocrusts and increases the abundance of moss biocrusts (De Guevara et al. 2018). Increased precipitation significantly promotes the growth of cyanobacteria and inhibits moss growth (Zelikovaet et al. 2012). A reduction in precipitation might prevent cyanobacterial biocrusts from reaching more mature successional stages (Fernandes et al. 2018). However, how microbial communities in biocrusts respond to climate change and whether the responses vary with biocrust development remains largely unclear.
Climate gradients are naturally formed along large geographic scales, and are ideal for studying the effects of climate change on terrestrial ecosystems and for various biogeographical studies, which aim to explore the spatial distribution patterns of biodiversity and community assembly processes (deterministic or stochastic processes) (Karimi et al. 2020; Chu et al. 2020; Feng et al. 2019). Community assembly processes are the basis for understanding species co-existence patterns and community composition (Dini-Andreote et al. 2015). Community dynamics are controlled by deterministic factors, such as environmental factors and stochastic processes, including birth, death, and dispersal (Ofiţeru et al. 2010). Furthermore, the dominance of deterministic and stochastic processes varies with geographic location and spatial scale (Shi et al. 2018). Microbial community diversity and composition change with biocrust succession. However, little is known about the assembly process of microbial communities in biocrusts at different successional stages.
In the context of global climate change, temperatures at high latitudes and elevations have increased significantly more than in other regions (IPCC 2007; Benito et al. 2011). The Qinghai-Tibetan Plateau, the highest and largest plateau on Earth (Shen et al. 2015) is experiencing rapid warming and precipitation regime changes. Considering its unique geographic location and significant ecological and production functions, the effects of climate change or human activities on biodiversity, vegetation composition, soil characteristics, and ecosystem functions have largely been studied in this area (Sun et al. 2020; Zhang et al. 2019, 2020). In this study, we sampled two types of BSCs with different succession stages from nine regions on the Qinghai-Tibetan Plateau that varied in altitude, annual temperature, and precipitation. Bacterial community diversity and assembly in the crust layer were measured using high-throughput sequencing, and soil nutrients underlying biocrusts were determined. The nine selected locations were across the north to south gradient along the eastern boundary of the Qinghai Province, covering the main grassland types (alpine steppe and alpine meadow) and showed significant climatic differences. To the best of our knowledge, this research is the most extensive investigation on a geographical scale of the microbial communities in the BSCs in the Qinghai-Tibet Plateau, which has significant application in the utilization of biocrust resources for ecosystem restoration. We hypothesized that 1) microbial community diversity increases with biocrust development, 2) microbial communities in biocrusts with different successional stages responds differently to climatic changes, 3) the community assembly process and co-occurrence network structure changes with biocrust succession, and 4) the ability to improve the underlying soil quality differs between the two biocrust types.