Biogeochemists have discovered that molybdenum, not phosphorus as previously thought, is the limiting nutrient that dictates the rate of bacterial nitrogen fixation in tropical soils (Nat. Geoscience, DOI: 10.1038/ngeo366). The finding is an important development for the study of global biogeochemical cycles, the scientists say, because molybdenum-limited nitrogen fixation could constrain plant growth and hence carbon dioxide uptake by plants, a major sink for atmospheric CO2 in the tropics. Nitrogen fixation is the biological conversion of nitrogen from air into ammonium that plants use as fertilizer. Nitrogenase enzymes produced by soil bacteria carry out the chemistry. These enzymes in turn have a cofactor that contains an iron-molybdenum cluster. Alexander R. Barron and Lars O. Hedin of Princeton University and coworkers tested weathered soils with low mineral content in a Panamanian rain forest by adding molybdenum, phosphate fertilizer containing trace amounts of molybdenum, and molybdenum-free phosphate. The molybdenum treatments significantly increased the rate of nitrogen fixation, whereas the pure phosphate treatment registered no change. The study confirmed the researchers' notion that past fertilizer studies pegging phosphorus as the rate-limiting nutrient may have been tainted by trace amounts of molybdenum.