UC Davis faculty member Benjamin Houlton of the department of land, air and water resources has co-published a research paper examining the role of the nitrogen cycle. This better understanding of the nitrogen cycle could lead to more accurate predictions of global climate change.
Titled “A unifying framework for dinitrogen fixation in the terrestrial biosphere,” the paper was published in the journal Nature on June 19.
“We were trying to address how to build better predictions in terms of climate change,” Houlton said.
The study focuses on the nitrogen cycle in tropical and temperate forests, specifically on trees that use nitrogen fixation, or the process of taking nitrogen directly from the atmosphere. These same trees often prosper when nitrogen is also abundant in the soil, but not when it is absent.
“The research of Dr. Houlton is important as it reveals fundamental mechanisms of biogeochemical cycling and tree nutrient uptake as it relates to environmental stresses on terrestrial ecosystems,” said Jan W. Hopmans, chair of the department of land, air and water resources.
Nitrogen levels determine how much carbon dioxide plants are able to extract from the atmosphere. As carbon dioxide is the primary gas involved in global warming, any process that affects the nitrogen levels in relation to plant growth will ultimately impact collective global temperatures.
“In general, impacts of climate change, particularly the earth’s warming, on biological systems continue to be largely unknown,” Hopmans said. “It is therefore that the plant’s functioning must be understood first before impacts – both negative and positive effects – on plants and trees and their diversity on climate warming can be better understood.”
The researchers concluded that the reason nitrogen-fixing trees were more abundant in areas in which the soil was nitrogen rich was due to two factors.
First, the temperature affects the activity of nitrogenase – an enzyme that catalyzes the reduction of molecular nitrogen in the nitrogen-fixation process of bacteria. According to the study, colder and more temperate climates require more of the enzyme in order to “fix” the present nitrogen. This high cost would ultimately counteract the advantage that temperate forests have in terms of nitrogen fixation resulting from nitrogen deficient soil.
Second, the presence of phosphorus in the soil of tropical forests explains why there is an abundance of nitrogen fixing plant life in those areas.
“The ability of N2 fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N2 fixation in phosphorus-limited tropical ecosystems,” according to the study.
“It turns out that these nitrogen fixing plants take excess nitrogen and invest it into enzyme systems in their roots,” Houlton said.
Houlton compared the efficiency of nitrogen fixing plants in warmer climates – and their ability to “invest” excess nitrogen through root systems – to principles generally observed in economics.
“We think plants will behave economically,” he said. “We just need to understand the playing field.”
Professor Peter Vitousek of Stanford University, Christopher Field of the Carnegie Institution of Global Ecology and Yingping Wang of Australia’s Commonwealth Scientific and Industrial Research Organization co-authored the paper.
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