Former Princeton University doctoral student Alexander Barron (left) and undergraduate Lisa Bennett (right) were part of a team of Princeton scientists that studied tropical rain forests in Panama. The trace element molybdenum has been found to support the lush growth of rain forests, including this giant tree of the species Ceiba pentandra, which is sacred in Mayan mythology and was considered the tree of life. (Credit: Lars Hedin)

PRINCETON (US)—Researchers have made a surprising discovery about the health of tropical rainforests. This vital part of the Earth’s ecosystem appears to rely on a rare trace element to capture the nitrogen fertilizer needed to remain lush and thriving.

Until now, scientists had thought that phosphorus was the key element supporting the prodigious expansion of rainforests, according to Lars Hedin, a professor of ecology and evolutionary biology at Princeton University who led the research. But an experiment testing the effects of various elements on test plots in lowland rainforests on the Gigante Peninsula in the Barro Colorado Nature Monument in Panama showed that areas treated with molybdenum withdrew more nitrogen from the atmosphere than other elements. Molybdenum is 10,000 times less abundant than phosphorus and other major nutrients in these ecosystems.

“It’s not what we were expecting,” says Hedin, who is also a professor in the Princeton Environmental Institute.

Molybdenum, the team found, is essential for controlling the biological conversion of nitrogen in the atmosphere into natural soil nitrogen fertilizer, which in turn spurs plant growth. “Just like trace amounts of vitamins are essential for human health, this exceedingly rare trace metal is indispensable for the vital function of tropical rainforests in the larger Earth system,” Hedin explains.

Previously, researchers knew little about rainforests’ capacity to absorb the greenhouse gas carbon dioxide. If molybdenum is central to the biochemical processes involved in the uptake of carbon dioxide, then there may be limits to how much carbon that tropical rainforests can absorb. This finding could have implications for global climate change policy.

The biological enzyme, nitrogenase, which converts atmospheric nitrogen into soil fertilizer, feeds on molybdenum, the researchers found. “Nitrogenase without molybdenum is like a car engine without spark plugs,” says Alexander Barron, the lead author on the paper and recent Princeton graduate who now is working on climate legislation in Congress.

Molybdenum, a lustrous, silvery metal, is found in soil, rock and sea water and in a range of enzymes vital to human health. Traces of the element have been found in Japanese swords dating back to the 14th century. In modern times, its high strength, good electrical conductivity, and anticorrosive properties have made molybdenum desirable as an element of rocket engines, radiation shields, light bulb filaments, and circuit boards.

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