Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils

Nitrogen fixation, the biological conversion of di-nitrogen to plant-available ammonium, is the primary natural input of nitrogen to ecosystems(1), and influences plant growth and carbon exchange at local to global scales(2-6). The role of this process in tropical forests is of particular concern, as these ecosystems harbour abundant nitrogen-fixing organisms(1,4) and represent one third of terrestrial primary production(4,7,8). Here we show that the micronutrient molybdenum, a cofactor in the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by free-living heterotrophic bacteria in soils of lowland Panamanian forests. We measured the fixation response to long-term nutrient manipulations in intact forests, and to short-term manipulations in soil microcosms. Nitrogen fixation increased sharply in treatments of molybdenum alone, in micronutrient treatments that included molybdenum by design and in treatments with commercial phosphorus fertilizer, in which molybdenum was a 'hidden' contaminant. Fixation did not respond to additions of phosphorus that were not contaminated by molybdenum. Our findings show that molybdenum alone can limit asymbiotic nitrogen fixation in tropical forests and raise new questions about the role of molybdenum and phosphorus in the tropical nitrogen cycle. We suggest that molybdenum limitation may be common in highly weathered acidic soils, and may constrain the ability of some forests to acquire new nitrogen in response to CO2 fertilization(9).