The Amazon rainforest is the Earth’s largest reservoir of plant and animal diversity, and it has been subjected to especially high rates of land use change, primarily to cattle pasture. This conversion has had a strongly negative effect on biological diversity, reducing the number of plant and animal species and homogenizing communities. We report here that microbial biodiversity also responds strongly to conversion of the Amazon rainforest, but in a manner different from plants and animals. Local taxonomic and phylogenetic diversity of soil bacteria increases after conversion, but communities become more similar across space. This homogenization is driven by the loss of forest soil bacteria with restricted ranges (endemics) and results in a net loss of diversity. This study shows homogenization of microbial communities in response to human activities. Given that soil microbes represent the majority of biodiversity in terrestrial ecosystems and are intimately involved in ecosystem functions, we argue that microbial biodiversity loss should be taken into account when assessing the impact of land use change in tropical forests.

Science has a critical role to play in guiding more sustainable development trajectories. Here, we present the Sustainable Amazon Network ( Rede Amazônia Sustentável , RAS): a multidisciplinary research initiative involving more than 30 partner organizations working to assess both social and ecological dimensions of land-use sustainability in eastern Brazilian Amazonia. The research approach adopted by RAS offers three advantages for addressing land-use sustainability problems: (i) the collection of synchronized and co-located ecological and socioeconomic data across broad gradients of past and present human use; (ii) a nested sampling design to aid comparison of ecological and socioeconomic conditions associated with different land uses across local, landscape and regional scales; and (iii) a strong engagement with a wide variety of actors and non-research institutions. Here, we elaborate on these key features, and identify the ways in which RAS can help in highlighting those problems in most urgent need of attention, and in guiding improvements in land-use sustainability in Amazonia and elsewhere in the tropics. We also discuss some of the practical lessons, limitations and realities faced during the development of the RAS initiative so far.

Here we show how agricultural practices by indigenous peoples as well as forest recovery relate to the structure and composition of Amazon soil bacterial communities. Soil samples were collected in different land use systems and bacterial community composition and diversity were explored by T-RFLP, cloning and sequencing, and data were analyzed with multivariate techniques. The main differences in bacterial community structure were related to changes in the soil attributes that, in turn, were correlated to land use. Community structure changed significantly along gradients of base saturation, [Al 3 þ ] and pH. The relationship with soil attributes accounted for about 31% of the variation of the studied communities. Clear differences were observed in community composition as shown by the differential distribution of Proteobacteria, Bacteroidetes, Firmicutes, Acidobacteria and Actinobacteria. Similarity between primary and secondary forest communities indicates the recovery of bacterial community structure during succession. Pasture and crop soil communities were among the most diverse, showing that these land use types did not deplete bacterial diversity under the conditions found in our sites.

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Anthropogenic pressures on tropical forests are rapidly intensifying, but our understanding of their implications for biological diversity is still very limited, especially with regard to soil biota, and in particular soil bacterial communities. Here we evaluated bacterial community composition and diversity across a gradient of land use intensity in the eastern Amazon from undisturbed primary forest, through primary forests varyingly disturbed by fire, regenerating secondary forest, pasture, and mechanized agriculture. Soil bacteria were assessed by paired-end Illumina sequencing of 16S rRNA gene fragments (V4 region). The resulting sequences were clustered into operational taxonomic units (OTU) at a 97% similarity threshold. Land use intensification increased the observed bacterial diversity (both OTU richness and community heterogeneity across space) and this effect was strongly associated with changes in soil pH. Moreover, land use intensification and subsequent changes in soil fertility, especially pH, altered the bacterial community composition, with pastures and areas of mechanized agriculture displaying the most contrasting communities in relation to undisturbed primary forest. Together, these results indicate that tropical forest conversion impacts soil bacteria not through loss of diversity, as previously thought, but mainly by imposing marked shifts on bacterial community composition, with unknown yet potentially important implications for ecological functions and services performed by these communities.

Because soil microbes drive many of the processes underpinning ecosystem services provided by soils, understanding how cropping systems affect soil microbial communities is important for productive and sustainable management. We characterized and compared soil microbial communities under restored prairie and three potential cellulosic biomass crops (corn, switchgrass, and mixed prairie grasses) in two spatial experimental designs -side-by-side plots where plant communities were in their second year since establishment (i.e., intensive sites) and regionally distributed fields where plant communities had been in place for at least 10 years (i.e., extensive sites). We assessed microbial community structure and composition using lipid analysis, pyrosequencing of rRNA genes (targeting fungi, bacteria, archaea, and lower eukaryotes), and targeted metagenomics of nifH genes. For the more recently established intensive sites, soil type was more important than plant community in determining microbial community structure, while plant community was the more important driver of soil microbial communities for the older extensive sites where microbial communities under corn were clearly differentiated from those under switchgrass and restored prairie. Bacterial and fungal biomasses, especially biomass of arbuscular mycorrhizal fungi, were higher under perennial grasses and restored prairie, suggesting a more active carbon pool and greater microbial processing potential, which should be beneficial for plant acquisition and ecosystem retention of carbon, water, and nutrients.

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The genera Exiguobacterium and Psychrobacter have been frequently detected in and isolated from polar permafrost and ice. These two genera have members that can grow at temperatures as low as À5 and À10 1C, respectively. We used quantitative PCR (Q-PCR) to quantify members of these genera in 54 soil or sediment samples from polar, temperate and tropical environments to determine to what extent they are selected by cold environments. These results were further analyzed by multiple linear regression to identify the most relevant environmental factors corresponding to their distribution. Exiguobacterium was detected in all three climatic zones at similar densities, but was patchier in the temperate and tropical samples. Psychrobacter was present in almost all polar samples, was at highest densities in Antarctica sediment samples, but was in very low densities and infrequently detected in temperate and tropical soils. Clone libraries, specific for the 16S rRNA gene for each genus, were constructed from a sample from each climatic region. The clone libraries were analyzed for a and b diversities, as well as for variation in population structure by using analysis of molecular variance. Results confirm that both genera were found in all three climatic zones; however, Psychrobacter populations seemed to be much more diverse than Exiguobacterium in all three climatic zones. Furthermore, Psychrobacter populations from Antarctica are different from those in Michigan and Puerto Rico, which are similar to each other.