UC Davis

The Amazon Rainforest is a global diversity hotspot that has experienced a significant level of deforestation over the past half century, primarily for the establishment of cattle pasture. Characterizing the impact of this large-scale ecosystem conversion on the composition and activity of the soil microbial community is crucial for understanding potentially consequential shifts in nutrient and greenhouse gas cycling, as well as adding to the body of knowledge concerning how tropical ecosystems respond to human disturbance. Research to date has shown that locally, communities of soil microorganisms tend to become more diverse upon conversion of forest to pasture. However, these communities undergo taxonomic homogenization at landscape-level spatial scales, mirroring the homogenization of plant communities across pastures. Microbial community structure is distinct between forest and pasture soil communities across several studies, and specific taxa, such as Firmicutes and Acidobacteria, show consistent association with pasture and forest soils, respectively. In addition, shifts in microbial community functions with pasture conversion have relevant impacts on both carbon and nitrogen cycling at the ecosystem scale: the abundance and diversity of methane-cycling prokaryotes shifts in conjunction with increased methane flux in pastures. Further, quantitation and community profiling of free-living nitrogen fixers has demonstrated that this functional group is favored in pastures and suggests that asymbiotic N2 fixation may be a significantly augmented process. While human-driven deforestation is continuing, a large percentage of once-converted pastures are undergoing the process of secondary forest succession. Assessment of microbial communities in secondary forests compared to primary forests and pastures suggests convergence toward a recovery of functionality and community composition with reforestation.