Biological nitrogen fixation (BNF) is the main pathway for new nitrogen (N) to enter ecosystems. The highest rates of BNF are found in tropical forests, but the majority of studies on BNF in tropical forests have been conducted along coastal regions, which receive high rates of sea-salt aerosols likely rich in molybdenum (Mo). Much evidence suggests that the low availability of Mo, a co-factor in the nitrogenase enzyme, can constrain BNF, even in forests that receive large inputs of Mo. Thus, Mo could be even more limiting to BNF in tropical forests isolated from atmospheric inputs. In Chapter 1, I modeled how sea-salt aerosols, mineral dust, and volcanoes distribute atmospheric Mo to terrestrial ecosystems. I then identified the southern Amazon basin as a region where atmospheric inputs and parent material weathering is low, and hypothesized that 1) Mo limitation of BNF is prevalent in this region, further exacerbated by low soil pH, and that 2) BNF rates would be low compared to other tropical forests. In Chapter 2, I conducted two long-term experiments where I added Mo, phosphorus (P), and lime (to raise soil pH) separately and together. I found that, free-living BNF did not respond strongly to Mo and P additions, likely because of high soil N availability that suppresses BNF. In the southeastern Amazon, interactions of drought, deforestation, and agricultural have led to increased forest fires. Early in forest succession after large N losses, BNF has been found to increase in many tropical forests to support secondary forest recovery. In Chapters 3 and 4, I asked if trees capable of hosting symbiotic BNF (N-fixing) in the Fabaceae family and associated symbiotic BNF, as well as free-living BNF, increase to support forest recovery post-fires. I found low rates of free-living and symbiotic BNF despite 1) large aboveground losses of N post-fire and 2) a large increase in the abundance of N-fixing trees. The low rates of BNF in the southeastern Amazon have important implications for understanding the role of BNF in tropical forests more generally, because large areas of tropical forest occur in lowland, interior basins on highly weathered oxisols.