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It is well established that land use change (LUC) can impact soil organic carbon (SOC) in tropical regions, but the long-term effects of LUC on soil quality and C cycling remain unclear. Here, we evaluated how LUC affects soil C cycling in the Amazon region using a 100-year observational chronosequence spanning primary forest-to-pasture conversion and subsequent secondary forest succession. We found a surprising increase in topsoil SOC concentrations 60years following conversion, despite major losses (> 85%) of forest-derived SOC within the first 25years. Shifts in molecular composition of SOC, identified with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, occurred in tandem with a significant decline in permanganate-oxidizable C (POXC) and β-glucosidase activity (perunit SOC), interpreted as a deceleration of soil C cycling after pasture grasses became the dominant sourceof C inputs to soil. Secondary forest succession caused rapid reversal to conditions observed under primary forest for β-glucosidase activity but not for SOC molecular composition (DRIFT spectroscopy),reflectinga long-lasting effect of LUC on soil C cycling. Our results show that rapid changes in the origin of SOC occur following deforestation with legacy effects on some indicators of C cycling (e.g. enzyme activity) but not others (e.g. molecular composition). This approach offers mechanistic understanding of LUC in the Amazon basin and can be used to help explain conflicting reports on how deforestation impacts SOC in the region.

© 2018 Kroeger, Delmont, Eren, Meyer, Guo, Khan, Rodrigues, Bohannan, Tringe, Borges, Tiedje, Tsai and Nüsslein. Deforestation in the Brazilian Amazon occurs at an alarming rate, which has broad effects on global greenhouse gas emissions, carbon storage, and biogeochemical cycles. In this study, soil metagenomes and metagenome-assembled genomes (MAGs) were analyzed for alterations to microbial community composition, functional groups, and putative physiology as it related to land-use change and tropical soil. A total of 28 MAGs were assembled encompassing 10 phyla, including both dominant and rare biosphere lineages. Amazon Acidobacteria subdivision 3, Melainabacteria, Microgenomates, and Parcubacteria were found exclusively in pasture soil samples, while Candidatus Rokubacteria was predominant in the adjacent rainforest soil. These shifts in relative abundance between land-use types were supported by the different putative physiologies and life strategies employed by the taxa. This research provides unique biological insights into candidate phyla in tropical soil and how deforestation may impact the carbon cycle and affect climate change.

© 2016 The Authors. Functional Ecology © 2016 British Ecological Society Land use change is a significant threat to biodiversity, particularly within tropical ecosystems, but the responses of microbial communities remain poorly understood. We used long-term plots established in multiple land use types in the Brazilian Amazon to examine the effect of land use change on soil fungal communities. We measured fungal richness and composition and identified factors associated with shifts in community composition across multiple land use types, including primary forest, two secondary forests and a chronosequence of differently aged pastures. Additionally, we used distribution patterns to estimate the niche breadth of fungal taxa in order to quantify changes in the relative abundance of generalists, or fungi with broad environmental tolerance, in response to land use change. Conversion of primary forest to pasture resulted in large reductions in fungal richness coupled with substantial changes in community composition. Generalist fungi were strongly favoured in all pasture sites, regardless of time since conversion. Distance to primary forests was the strongest correlate of community composition in pastures, indicating that primary forests can act as reservoirs for recolonization by forest-associated fungi. The two secondary forests showed variable patterns of richness, composition and the overall abundance of generalist fungi, suggesting that community recovery is stochastic. Fungal community response to land use change mirrors patterns observed in macroscopic organisms, which indicates that the increased prevalence of generalist taxa is a consistent response to disturbance across broad taxonomic groups. A lay summary is available for this article.