環境資訊中心外電；姜唯 翻譯；林大利 審校；稿源：ENS
北美三所大學的科學家發現植物的固氮方式不止一種，當固氮所需主要微量元素缺乏時，自然界有其「B計畫」。
氮元素佔地球大氣層的78％和人體的3％，是所有生物都不可或缺的元素，但植物和自然系統要獲得氮卻並不容易。
大多數生物不能直接利用大氣中的氮，而是透過土壤和水中的特殊微生物將氮轉化為生物可以直接利用的氨，這個過程就是固氮作用。
正在進行植物固氮研究的試驗田。 （CC BY-NC-ND 2.0）
例如在農業中，種植大豆和其他有助於固氮的豆類，可以恢復貧瘠土壤的肥力。
問題來了。微生物固氮時使用了一種複雜的蛋白質叫做「固氮酶」，其中富含金屬。過去的研究聚焦於含有稀有金屬「鉬」的固氮酶。鉬不會以游離金屬的形式存在自然界，只以各種氧化態存在於礦物中。
由於鉬很稀有，人們注意到這是土地固氮的自然限制。科學家擔心，在人為干擾、人口成長而需要開墾貧瘠土地的情況下，鉬的稀有性會阻礙生態系統恢復肥沃度的能力。
現在，普林斯頓大學、杜克大學和加拿大魁北克舍布魯克大學的科學家合作，發現當鉬缺乏時，至少還有另外一種金屬可以完成固氮工作。
科學家們在加拿大的北寒林中進行研究工作，發現金屬釩可以催化整個生態系統的固氮活動，尤其是在自然氮來源有限的北部地區。
「這項研究改變了我們對微量營養素如何控制生態系統氮素狀況和肥力的認知。我們需要從養分預算、循環和生物多樣性方面更深入了解固氮作用。」研究資深作者、普林斯頓環境研究所地球科學助理教授張欣寧（音譯）說。
張欣寧說：「研究結果顯示，透過固氮作用進入北寒林（boreal forest）的氮量可能被大大低估了。這是關於森林養分需求的重大發現，因為森林生態系統是人為碳的重要匯集區。」
荷蘭烏特勒支大學科學家2018年7月的一項研究中警告，地球上的鉬資源可能會在50至100年內耗盡。該研究發表在《資源保護和循環利用》期刊上，標題是「」。
同時烏特勒支大學的研究也指出，由於轉型零化石燃料能源生產必須要用到鉬，鉬的使用量無可避免會增加。尋找替代品、提高材料效率和減少耗損的空間有限，只有尾礦中的鉬損失可以減半。
科學家說，要實現永續利用，鉬的回收率必須從目前的約20％增加到至少80％。有必要採取政策性的措施，「因為不確定是否能靠自由市場價格機制及時達到所需的鉬回收率。」
普林斯頓大學研究第一作者、博士後科學家Romain Darnajoux說，只有當環境鉬含量低時，才會發生釩基固氮，「當環境變化時，自然界似乎演化出備用方案來維持生態系統的肥力。」
「每個氮循環步驟都涉及一種酶，這些酶需要特定的微量金屬才能發揮作用，」Darnajoux說，到目前為止，相關研究一直集中在鉬和鐵上，釩基固氮酶幾乎沒有被注意到。
研究結果顯示，目前透過固氮進入北寒林的氮估計值太低，這會低估植物生長所需的氮量。
儘管北寒林不像人口稠密都會區吸引眾多遊客，但人類活動仍然會透過大氣中的氮和鉬、釩等金屬污染物的移動，影響森林的肥力。
張欣寧說：「人類活動大幅改變了空氣品質，甚至可能顯著影響地處偏遠的生態系統。研究結果顯示空氣污染改變微量營養素和主要營養素動態是一個很重要的問題。」
這些發現可能有助於發展更精確的氣候模型，目前的模型模擬全球氮素在陸地、海洋和大氣間流動時，並未明確包含鉬或釩的資訊。
這份研究於2019年11月11日。
Fixation on Nitrogen PRINCETON, New Jersey, November 11, 2019 (ENS)
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.
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