Nature Geosci. doi: 10.1038/ngeo366 (2008)

A lack of the silvery metal molybdenum limits nitrogen fixation in tropical soils. The finding comes from the work of Lars Hedin at Princeton University in New Jersey and his colleagues, and calls into question the role phosphorus was thought to have in the process.

Hedin and his team looked at the activity of nitrogenase — the enzyme that converts atmospheric nitrogen into ammonium, which plants can use — in the top layer of soil in a tropical forest floor in Panama. The plots had been given fertilizers rich in nitrogen, phosphorus or micronutrients including molybdenum over a period of seven years. The researchers also measured the short-term effects of molybdenum and phosphorus on nitrogenase activity.

Results from both experiments suggest that molybdenum alone can trigger nitrogenase activity, and that contamination by this element might underlie the efficacy of commercial phosphorus fertilizers, Hedin says.