Soils harbour some of the most diverse microbiomes on Earth and are essential for both nutrient cycling and carbon storage. To understand soil functioning, it is necessary to model the global distribution patterns and functional gene repertoires of soil microorganisms, as well as the biotic and environmental associations between the diversity and structure of both bacterial and fungal soil communities. Here we show, by leveraging metagenomics and metabarcoding of global topsoil samples (189 sites, 7,560 subsamples), that bacterial, but not fungal, genetic diversity is highest in temperate habitats and that microbial gene composition varies more strongly with environmental variables than with geographic distance. We demonstrate that fungi and bacteria show global niche differentiation that is associated with contrasting diversity responses to precipitation and soil pH. Furthermore, we provide evidence for strong bacterial-fungal antagonism, inferred from antibiotic-resistance genes, in topsoil and ocean habitats, indicating the substantial role of biotic interactions in shaping microbial communities. Our results suggest that both competition and environmental filtering affect the abundance, composition and encoded gene functions of bacterial and fungal communities, indicating that the relative contributions of these microorganisms to global nutrient cycling varies spatially.

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SUMMARYThe arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck and Smith was grown in symbiosis with Cucmnis sativus L. cv. Aminex (Fl hybrid) in mesh hags surrounded by a sand-filled hyphal compartment (HC), allowing only the fungal hyphae to protrude into the HC. The hyphae in the HC were supplied with '°N-labelled NH,* or NOj" after 60 d (expt 1). Following a 48 h labelling period, the sand was removed from the HC and the hyphae extracted. In another experiment (expt 2), the hyphae were extracted from the sand before being incubated in vitro in a nutrient solution containing '""'N-labelled NH^* for 15 h. The hyphal material was incubated in a 0 or 2'5 mM solution of the GOGAT-inhibitor albizzine prior to labelling. In both experiments the hypha! content of free amino acids and fatty acids w^ere measured as well as the amino acid ^°N enrichment.Asparagine was the hyphal amino acid measured in highest concentration followed by glutamine, glutamate, aspartate and alanine. Ornithine, serine and glycine were detected in lower concentrations, but y-ammobutyric acid and citrulline were not detectable. This pattern was the same in both experiments and was unaffected by the type of N applied to the hyphae or by preincubation with albizzine, although the amino-acid concentration decreased considerably in expt 2 compared with expt 1. Both NH^*-N and NO3~-N were assimilated into amino acids but the levels of '°N enrichment following application of NO3~ were much lower than those following application of NH^^ indicating that the latter was more readily assimilated. Albizzine decreased the hyphal amino acid concentration by c. 30 % (without affecting the ^°N enrichment of the individual amino acids) indicating that the AM-fungal hyphae might possess a GS-GOGAT enzyme system for assimilation of inorganic N. The fattyacid profiles (especially phospholipid fatty acids 16:lw5 and 20:5) obtained from the hyphae of G. intraradices showed that contamination of the samples by fungi other than G. intraradices and bacteria was insignificant, and confirmed the usefulness of specific fatty-acid measurement to estimate soil AM-fungal content.

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Turnover rates of arbuscular mycorrhizal (AM) fungi may influence storage of soil organic carbon (SOC). We examined the longevity of AM hyphae in monoxenic cultures; and we also used 13 C incorporation into signature fatty acids to study C dynamics in a mycorrhizal symbiosis involving Glomus intraradices and Plantago lanceolata. 13 C enrichment of signature fatty acids showed rapid transfer of plant assimilates to AM fungi and a gradual release of C from roots to rhizosphere bacteria, but at a much slower rate. Furthermore, most C assimilated by AM fungi remained 32 days after labelling. These findings indicate that 13 C labelled fatty acids can be used to track C flux from the atmosphere to the rhizosphere and that retention of C in AM fungal mycelium may contribute significantly to SOC.