"Our results were quite unexpected," said Hedin, a professor of ecology and evolutionary biology. "We discovered that the trace element molybdenum often was the limiting nutrient, not phosphorus, as most theories would predict."
In phosphorus-poor soils, nitrogen fixation did not increase with the addition of phosphorus alone-or, for that matter, molybdenum alone-but only responded when both phosphorus and molybdenum were added together.
"The chemistry of the leaf litter seems to lock up the molybdenum and make it unavailable to bacteria," Wurzburger said. "Therefore, no matter how abundant or poor phosphorus is in the environment, it's molybdenum that seems to present a consistent constraint for nitrogen-fixing bacteria." [emphasis mine]
That's interesting. Thanks.