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Determining the influence of climate in driving the global distribution of soil microbial communities is fundamental to help predict potential shifts in soil food webs and ecosystem functioning under global change scenarios. Herein, we used a global survey including 80 dryland ecosystems from six continents, and found that the relative abundance of ecological clusters formed by taxa involved in bacteria-fungi and bacteria-cercozoa bipartite networks was highly sensitive to changes in temperature and aridity. Importantly, such a result was maintained when controlling for soil, geographical location and vegetation attributes, being pH and soil organic carbon important determinants of the relative abundance of the ecological clusters. We also identified potential global associations between important soil microbial taxa, which can be useful to support the conservation of terrestrial ecosystems under global change scenarios. Our results suggest that increases in temperature and aridity such as those forecasted for the next decades in drylands could potentially lead to drastic changes in the community composition of functionally important bipartite networks within soil food webs. This could have important but unknown implications for the provision of key ecosystem functions and associated services driven by the organisms forming these networks if other taxa cannot cope them.This article is part of the theme issue ‘Ecological complexity and the biosphere: the next 30 years’.

This study was conducted within the framework of the DFG project SPP2089 “Rhizosphere Spatiotemporal Organization – a Key to Rhizosphere Functions”.Different plant species select for individual subsets of bulk soil microbial communities within root systems. The fast variability of root environments implies that roots constitute highly dynamic habitats. Rapid root elongation, combined with widely varying quality and quantity of rhizodeposition between different root regions, lead to continuously changing conditions for colonizing microorganisms. As the microbiome concept implies a rather static outcome of the microbial assembly, it raises the question as to where and how the dynamic transition of a microbial bulk soil community into a plant species-specific rhizosphere microbiome is taking place.To investigate the assembly of communities of prokaryotes and their microbial predators (Cercozoa, Rhizaria; protists) along the longitudinal root axis of maize (Zea mays L.), plants were grown in an agricultural loamy soil. Rhizosphere soil was sampled at distinct locations along roots. Diversity and co-occurrence of rhizosphere microbiota along the root axis were tracked by high-throughput sequencing, diversity measures and network analyses.High variation in beta diversity at root tips and the root hair zone indicated substantial randomness of community assembly. Deterministic processes of community assembly were revealed by low variability of beta diversity, changes in network topology, and the appearance of regular phylogenetic co-occurrence patterns in bipartite networks between prokaryotes and their microbial predators. Deterministic processes were most robust in regions with fully developed lateral roots, suggesting that a consistent rhizosphere microbiome finally assembled. For the targeted improvement of microbiome function, such knowledge on the processes of microbiome assembly on roots and its temporal and spatial variability is of crucial importance.

Biological soil crusts (biocrusts) accommodate diverse communities of phototrophic and heterotrophic microorganisms. Heterotrophic protists have critical roles in the microbial food webs of soils, with Cercozoa and Endomyxa often being dominant groups. Still, the diversity, community composition, and functions of Cercozoa and Endomyxa in biocrusts have been little explored. In this study, using a high-throughput sequencing method with taxon-specific barcoded primers, we studied cercozoan and endomyxan communities in biocrusts from two unique habitats (subarctic grassland and temperate dunes). The communities differed strongly, with the grassland and dunes being dominated by Sarcomonadea (69%) and Thecofilosea (43%), respectively. Endomyxa and Phytomyxea were the minor components in dunes. Sandonidae, Allapsidae, and Rhogostomidae were the most abundant taxa in both habitats. In terms of functionality, up to 69% of the grassland community was constituted by bacterivorous Cercozoa. In contrast, cercozoan and endomyxan communities in dunes consisted of 31% bacterivores, 25% omnivores, and 20% eukaryvores. Facultative and obligate eukaryvores mostly belonged to the families Rhogostomidae, Fiscullidae, Euglyphidae, Leptophryidae, and Cercomonadidae, most of which are known to feed mainly on algae. Biocrust edaphic parameters such as pH, total organic carbon, nitrogen, and phosphorus did not have any significant influence on shaping cercozoan communities within each habitat, which confirms previous results from dunes.