Nitrogen (N) fixation, the biologically mediated conversion of atmospheric N₂ to plant-available NH₄⁺, is the primary natural input of N to ecosystems with implications for plant growth and ecosystem-atmosphere carbon exchange from local to global scales.  While there has been progress in understanding N fixation in marine ecosystems, we know comparably little about factors that control this process in terrestrial ecosystems.  Tropical forests are of particular concern as they contribute roughly one third of N fixed on land, harbor one third of land primary production, and play a critical role in the Earth's climate system.  Here we report direct experimental demonstration that the soil micronutrient molybdenum (Mo) controls heterotrophic N fixation in tropical forests that grow on weathered soils.  We show that, in lowland Panamanian forests, N fixation increased in response to long-term fertilization with either phosphorus (P) or micronutrients, but decreased in response to N.  Direct additions confirmed that the trace-element molybdenum (Mo) caused the micronutrient effect.  Analyses further showed that Mo contamination of the P fertilizer may act as a "hidden treatment" that adequately explains the observed P effect.   Our findings constitute the first-ever experimental observation that Mo can limit N fixation in tropical forests, and suggest that Mo in P fertilizer may cause Mo limitation of fixation to be mistaken for P limitation.