Precipitation and topsoil attributes determine the species diversity and distribution patterns of crustal communities in desert ecosystems

Despite the important role of biological soil crusts in desert ecosystems, the responses of the crustal composition and distribution to changes in both climatic and edaphic factors at different spatial scales are not well understood. The goal of our research was to study the drivers of crustal community diversity at different scales, determine how crustal diversity and distribution patterns are related to abiotic and biotic factors, and identify the characteristics of crustal communities along a precipitation gradient. We investigated the distribution and characteristics of crustal communities based on 457 soil samples that were randomly collected from six desert regions of northern China. These regions represent a precipitation gradient from 450 mm in the east to 80-100 mm in the west. The measured environmental variables included soil moisture, pH, clay and silt content, soil organic carbon (SOC), total N, total P, total K, CaCO3 content, annual and perennial plant cover and crustal variables, including species richness, crustal cover and biomass. An ANOVA analysis, a stepwise regression and a redundancy analysis were used to analyse and interpret the response of biological soil crusts to environmental factors at two spatial scales. Our results showed that the moss distribution and biomass were strongly related to the topsoil moisture content. The highest diversity of cyanobacteria and algae was observed at sites with relatively dry topsoil, and lichen diversity was promoted at sites with fine-textured soils. In addition, the cover and biomass of moss were positively correlated with perennial plant cover and higher topsoil moisture, which was expected because plant cover provides shade for the moss. The cover and biomass of lichens, cyanobacteria and algae were positively correlated with that of annual plants because these plants may create an ideal habitat and provide nutrients. The physiochemical properties of the topsoil greatly influenced the distribution pattern of the crustal communities at the regional scale. The species richness and biomass of the crustal mosses were positively correlated with precipitation, whereas the species richness and biomass of cyanobacteria and algae were negatively correlated with precipitation at the landscape scale. Rainfall largely determined the species richness and cover of the crustal cyanobacteria and mosses, whereas soil properties had a greater impact on the crustal lichens. Future changes to rainfall regimes could result in the conversion of biological soil crusts and further contribute to changes in the structure and functioning of desert ecosystems.