July 18, 2008

Predicting climate change; Prof solves nitrogen paradox, portending improved accuracy

By Sylvia Wright

	Benjamin Houlton, an assistant professor of land, air and water resources, is pictured in his lab in the Plant and Environmental Sciences Building. Karin Higgins/UC Davis

A team of researchers led by a first-year UC Davis faculty member has resolved a longstanding paradox in the plant world, which should lead to far more accurate predictions of global climate change.

The journal Nature recently published a paper on the research.

The paradox centers on puzzling aspects of the nitrogen cycle in temperate and tropical forests. Defying the usual laws of supply and demand, trees capable of extracting nitrogen directly from the atmosphere (a process called nitrogen fixation) often thrive where it is readily available in the soil, but not where it is in short supply.

Nitrogen is an essential nutrient for all life on Earth, and determines how much carbon dioxide plants (and entire ecosystems) can absorb from the atmosphere, said Benjamin Houlton, an assistant professor who took the lead in writing the paper. Because carbon dioxide is the principal greenhouse gas causing global warming, any process that changes the amount of nitrogen available for plant growth will affect global temperatures.

But any serious attempt to consider the impact of nitrogen on climate change has been limited by a lack of understanding of the global pattern of nitrogen fixation, Houlton said.

You would expect that nitrogen-fixing species would have a competitive advantage in ecosystems where nitrogen is in low supply, but not where nitrogen is abundant, because fixation is energetically very costly to an organism. And thats the way ecologists have found it works in the open ocean and in lakes, he continued.

But in mature temperate forests, where the soils have limited amounts of nitrogen, nitrogen-fixing tree species are scarce. And in the tropical lowland forests, which are nitrogen-rich, nitrogen-fixing trees often are abundant. We asked why.

The researchers found the explanation lies in the key roles played by two other factors: temperature and the abundance of another key element, phosphorus.

Temperature, they determined, affects the activity of a nitrogen-fixing enzyme called nitrogenase. In cooler, temperate climates, more of the enzyme is needed to fix a given amount of nitrogen. This higher cost would offset the benefit of nitrogen fixation in temperate forests, despite low-nitrogen soils.

In tropical forests, it is 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, Houlton said. 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.