One part of the solution to the puzzle, the researchers found, is the nitrogen-fixing enzyme nitrogenase. A survey of diverse species and bacterial strains across different latitudes and environments showed the strong influence of temperature on the enzyme's activity. A consequence is that in cooler climates more of the enzyme is needed to fix a given amount of nitrogen. The high cost of producing the enzyme offsets the benefit of nitrogen fixation in temperate forests, despite low-nitrogen soils.
In tropical forests, it's the link between nitrogen and phosphorus that explains the abundance of nitrogen-fixing species.
"Many tropical [forest] soils are severely depleted in phosphorus, even where nitrogen is relatively abundant," says Houlton. "The extra nitrogen added to the soil by nitrogen-fixers helps mobilize phosphorus, making it easier for roots to absorb. That stimulates the growth of these plant species and puts them at a competitive advantage, despite the energetic cost of nitrogen fixation. It's really quite striking how simple the economics of nitrogen fixation fall out, once you consider the link between the nitrogen and phosphorus cycles in the tropics"
"Put together, these two factors give us a coherent picture of what was formerly a very enigmatic distribution pattern for nitrogen-fixing trees in plant communities," says Field, director of the Department of Global Ecology. "The more we understand about these essential ecological processes, the better we'll be able to manage Earth's ecosystems in the coming decades, especially in the face of unprecedented climate change."
|Contact: Christopher Field|