Soil bacteria and fungi play key roles in the functioning of terrestrial ecosystems, yet our understanding of their responses to climate change lags significantly behind that of other organisms. This gap in our understanding is particularly true for drylands, which occupy ∼41% of Earth´s surface, because no global, systematic assessments of the joint diversity of soil bacteria and fungi have been conducted in these environments to date. Here we present results from a study conducted across 80 dryland sites from all continents, except Antarctica, to assess how changes in aridity affect the composition, abundance, and diversity of soil bacteria and fungi. The diversity and abundance of soil bacteria and fungi was reduced as aridity increased. These results were largely driven by the negative impacts of aridity on soil organic carbon content, which positively affected the abundance and diversity of both bacteria and fungi. Aridity promoted shifts in the composition of soil bacteria, with increases in the relative abundance of Chloroflexi and α-Proteobacteria and decreases in Acidobacteria and Verrucomicrobia. Contrary to what has been reported by previous continental and global-scale studies, soil pH was not a major driver of bacterial diversity, and fungal communities were dominated by Ascomycota. Our results fill a critical gap in our understanding of soil microbial communities in terrestrial ecosystems. They suggest that changes in aridity, such as those predicted by climatechange models, may reduce microbial abundance and diversity, a response that will likely impact the provision of key ecosystem services by global drylands.bacteria | fungi | climate change | arid | semiarid

Abstract1. Dryland vegetation is characterized by discrete plant patches that accumulate and capture soil resources under their canopies. These "fertile islands" are major drivers of dryland ecosystem structure and functioning, yet we lack an integrated understanding of the factors controlling their magnitude and variability at the global scale.2. We conducted a standardized field survey across 236 drylands from five continents.At each site, we measured the composition, diversity and cover of perennial plants.Fertile island effects were estimated at each site by comparing composite soil

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Expansion of agricultural land use has increased emission of greenhouse gases, exacerbating climatic changes. Most agricultural soils have lost a large portion of their antecedent soil organic carbon storage, becoming a source of atmospheric carbon-dioxide. In addition, agricultural soils can also be a major source of nitrous oxide and methane. Adoption of conservation agricultural practices may mitigate some of the adverse impacts of landuse intensification. However, optimal implementation of these practices is not feasible under all physical and biotic conditions. Of a wide range of conservation practices, the most promising options include agroforestry systems and soil application of biochar, which can efficiently sequester large amounts of carbon over the long-run. In addition, these practices also increase agronomic productivity and support a range of ecosystem services. Payments to farmers and land managers for sequestrating carbon and improving ecosystem services is an important strategy for promoting the adoption of such practices, aimed at mitigating climate change while decreasing environmental footprint of agriculture and sustaining food security.