不只靠鉬、也能靠釩 科學家發現自然界固氮有「B計畫」

Scientists at three North American universities have discovered one of nature's "backup methods" for converting nitrogen into plant nutrients. Research published today in the journal "Proceedings of the National Academy of Sciences," shows that the process may be more resilient than previously known.

Although the element nitrogen is essential for all living organisms – it makes up 78 percent of Earth's atmosphere and three percent of the human body – it is not easy for plants and natural systems to access.

Nitrogen in the atmosphere is not directly usable by most living things. Instead, specialized microbes in soils and bodies of water convert nitrogen into ammonia, a form of nitrogen that life can easily access, through a process called nitrogen fixation.

In agriculture, for example, soybeans and other beans and peas that facilitate nitrogen fixation can be planted to restore the fertility of depleted soils.

But there's a catch. Microbial nitrogen "fixers" incorporate a complex protein called nitrogenase that contains a metal-rich core. Until now, research has focused on nitrogenases containing a scarce metal, molybdenum. Molybdenum does not occur naturally as a free metal on Earth; it is found only in various oxidation states in minerals.

The scarcity of molybdenum has raised concerns about the natural limits of nitrogen fixation on land. Scientists worry that the lack of molybdenum may obstruct nature's capacity to restore ecosystem fertility after human disturbances, or as people search for arable land to feed a growing population.

Now, researchers at Princeton University, working with scientists at Duke University and at the University of Sherbrooke in Québec, Canada, have found evidence that at least one other metal can enable nitrogen fixation when molybdenum is scarce.

Working in Canada's boreal forest, the researchers found that nitrogen fixation at an ecosystem scale can be catalyzed by the metal vanadium, particularly in northern regions with limited natural nitrogen inputs.

"This work prompts a major revision of our understanding of how micronutrients control ecosystem nitrogen status and fertility. We need to know more about how nitrogen fixation manifests in terms of nutrient budgets, cycling, and biodiversity," said the paper's senior author Xinning Zhang, assistant professor of geosciences and the Princeton Environmental Institute, PEI.

"One consequence of this finding is that current estimates of the amount of nitrogen input into boreal forests through fixation may be significantly underestimated," said Zhang.

"This is a major issue for our understanding of nutrient requirements for forest ecosystems, which currently function as an important sink for anthropogenic carbon," she explained.

Earth's molybdenum resources could be exhausted within 50 to 100 years, warns a July 2018 study by scientists at Utrecht University in The Netherlands. Published in the journal "Resources, Conservation and Recycling," the study is titled, "Molybdenum resources: Their depletion and safeguarding for future generations."

At the same time, an increase of molybdenum use is inevitable because of the role of molybdenum in the transition to a fossil-fuel-free energy generation, concluded the Utrecht group, led by M.L.C.M. Henckens.

The potentials of substitution, material efficiency improvement and dissipation reduction are limited, but molybdenum losses in tailings can be halved, Henckens wrote.

For a sustainable extraction rate, molybdenum recycling must be increased from about 20 percent at present to at least about 80 percent in the future, the Utrecht scientists wrote.

They deemed policy measures to be necessary, "because it is very uncertain whether the required molybdenum recycling rate will be achieved in time just by the free market price mechanism."

Back at Princeton, first author Romain Darnajoux, a postdoctoral scientist in Zhang's research group, said the investigators found that vanadium-based nitrogen fixation kicks in only when environmental molybdenum levels are low.

"It would seem that nature evolved backup methods to sustain ecosystem fertility when the environment is variable," Darnajoux said.

"Every nitrogen-cycle step involves an enzyme that requires particular trace metals to work, Darnajoux explained. Until now, research has focused on molybdenum and iron, while a vanadium-based nitrogenase that also exists has been "largely ignored," he said.

The researchers' results suggest that the current estimates of nitrogen input into boreal forests through fixation are woefully low, which would underestimate the nitrogen demand for robust plant growth, Darnajoux said.

Though these northern boreal forests do not see as many human visitors as even the most lightly populated metropolis, human activities can still have major impacts on forest fertility through the atmospheric transport of air pollution loaded with nitrogen and metals such as molybdenum and vanadium.

"Human activities that substantially change air quality can have a far-reaching influence on how even remote ecosystems function," Zhang said.

"The findings highlight the importance of air pollution in altering micronutrient and macronutrient dynamics," she said. 

The findings could help in the development of more accurate climate models, which do not explicitly contain information on molybdenum or vanadium in simulations of the global flow of nitrogen through land, ocean, and atmosphere.

The paper, "Molybdenum threshold for ecosystem-scale alternative vanadium nitrogenase activity in boreal forests," was published by the "Proceedings of the National Academy of Sciences" online in-advance of print on November 11, 2019.