發現氮在全球的新來源

原文網址：https://www.ucdavis.edu/news/new-source-global-nitrogen-discovered

發現氮在全球的新來源

岩石或許含有碳封存的關鍵，並且可以讓科學家更加準確地預測全球氣候

數個世紀以來，主流科學理論認為地球上可供植物利用的氮都是來自於大氣。但是加州大學戴維斯分校近日發表的研究，卻顯示其中有超過四分之一其實是來自於地球的岩床。

這篇4月6日發表於期刊《科學》的研究，發現自然界的生態系中，將近26%的氮來源是岩石，其餘部分才是來自大氣。

在此研究之前，科學家並不確定全球陸域生態系中的氮是從什麼地方輸入。作者表示這種新發現的氮來源也能助長陸地上的碳循環，使得生態系將更多排放至大氣中的碳抽取出來。由於氣候變遷的預測和我們對碳循環的瞭解程度有密切關係，因此這項研究也能大幅增加預測準確度。

「我們的研究顯示產氮風化作用(nitrogen weathering)對於世界各地的土壤和生態系來說是一種重要的氮來源。」研究共同主要作者Ben Houlton表示。他是加州大學戴維斯分校土地、空氣與水資源學系的教授，同時也是該校謬爾研究所的主任。「這和數個世紀以來組成環境科學基石的經典理論相悖。我們認為這些氮或許能讓森林和草原比過去以為的封存更多從化石燃料排出的二氧化碳。」

關鍵是風化作用

生態系需要氮和其他養分才能吸收二氧化碳帶來的汙染，不過在植物和土壤之中這些物質可用的量是有限度的。若有大量的氮從岩石中產生，就有助於解釋北方針葉林之類的自然生態系為何能夠吸收大量的二氧化碳。

然而，並非所有岩石都可以淋溶出氮。岩石中有多少氮可以被利用取決於風化作用。風化作用有兩種類型：其中一種是物理風化，像是透過構造運動所造成的風化；另一種則是化學風化，像是礦物跟水反應時發生的風化。

這便是在不同地區和地形之下，產氮風化作用的強度會有所差異的主因。研究表示非洲大部分地區的岩床都缺乏富含氮的岩石，而某些北方的高緯度地區則擁有程度最強的產氮風化作用。他們估計喜瑪拉雅山和安地斯山之類的高山地區是岩石風化產生的氮的重要來源，如同它們對於全球風化速率和氣候而言也是相當重要。另外，發生在草原、苔原、沙漠和林地的產氮風化作用速率也相當可觀。

地質和碳封存

繪出岩石中的養分多寡如何分布以瞭解它們吸收碳的能力，有助於推動保育政策的規劃。產氮岩石風化作用速率較高的地區或許可以封存更多的碳。

「哪些系統可以吸收二氧化碳哪些不行，或許有很大一方面是取決於地質條件。」Houlton表示，「從碳封存的角度進行考量時，地球本身的地質就能指引我們進行保育時的決策。」

神秘的差距

研究也闡明了「氮的消失問題」。數十年來科學家都知道土壤和植物中聚積的氮超過僅用大氣所能解釋的，然而他們無法指出到底是缺漏了哪一部份。

「我們證明了這個關於氮的矛盾現象的解答就寫在石頭當中。」共同主要作者Scott Morford表示。他於研究進行時為加州大學戴維斯分校的研究生。「岩石中不僅含有足夠的氮，它們的分解速率也快到足以可以解釋這道神秘的差距。」

在先前的研究中，研究團隊分析了加州北部克拉馬斯山脈的古老岩石樣品，他們發現這些岩石和周遭的樹木當中都含有大量的氮。以上述工作為基礎，作者在本篇研究中分析了整個地球的氮平衡和地球化學指標，並建立產氮風化作用的空間分布模型，進而估算在全球尺度下岩石中有多少氮可以被利用。

對於農藝和園藝人員這些依靠自然或合成產生的氮來種植作物的人來說，研究人員表示這項成果無法直接運用到他們身上。先前有些研究推論地下水中氮的背景值可以追溯至以岩石為來源，但還需要深入研究才能更加瞭解究竟量有多少。

改寫教科書

「教科書會因為這些結果而需要重新編寫。」美國國家科學基金會環境生物學部門的計畫主任Kendra McLauchlan表示。本研究經費有一部份來自該組織。「雖然以往已經有跡象顯示植物可以利用來自於岩石的氮，但這項研究確實粉碎了植物可用的氮基本上都是來自於大氣的經典理論。氮不僅是地球上最重要的限制養分，同時也可以造成相當嚴重的污染，因此瞭解自然情況下氮的供給與消耗受什麼因素影響相當重要。人類目前是利用大氣中的氮來製造肥料，以持續供給食物給全世界。對於這種不可或缺的養分所進行的研究，將會因為如此重大的發現而展開新的紀元。」

另一名加州大學戴維斯分校土地、空氣與水資源學系的教授Randy Dahlgren也參與了這項研究。

本研究的經費來自於美國國家科學基金會的地球科學部和環境生物學部，以及Andrew W. Mellon基金會。

New source of global nitrogen discovered

Rocks could hold key to carbon storage and improved global climate projections

By Kat Kerlin 

For centuries, the prevailing science has indicated that all of the nitrogen on Earth available to plants comes from the atmosphere. But a study from the University of California, Davis, indicates that more than a quarter comes from Earth’s bedrock.

The study, to be published April 6 in the journal Science, found that up to 26 percent of the nitrogen in natural ecosystems is sourced from rocks, with the remaining fraction from the atmosphere.

Before this study, the input of this nitrogen to the global land system was unknown. The discovery could greatly improve climate change projections, which rely on understanding the carbon cycle. This newly identified source of nitrogen could also feed the carbon cycle on land, allowing ecosystems to pull more emissions out of the atmosphere, the authors said.

“Our study shows that nitrogen weathering is a globally significant source of nutrition to soils and ecosystems worldwide,” said co-lead author Ben Houlton, a professor in the UC Davis Department of Land, Air and Water Resources and director of the UC Davis Muir Institute. “This runs counter the centuries-long paradigm that has laid the foundation for the environmental sciences. We think that this nitrogen may allow forests and grasslands to sequester more fossil fuel CO₂ emissions than previously thought.”

Weathering is key

Ecosystems need nitrogen and other nutrients to absorb carbon dioxide pollution, and there is a limited amount of it available from plants and soils. If a large amount of nitrogen comes from rocks, it helps explain how natural ecosystems like boreal forests are capable of taking up high levels of carbon dioxide. 

But not just any rock can leach nitrogen. Rock nitrogen availability is determined by weathering, which can be physical, such as through tectonic movement, or chemical, such as when minerals react with rainwater.

That’s primarily why rock nitrogen weathering varies across regions and landscapes. The study said that large areas of Africa are devoid of nitrogen-rich bedrock while northern latitudes have some of the highest levels of rock nitrogen weathering. Mountainous regions like the Himalayas and Andes are estimated to be significant sources of rock nitrogen weathering, similar to those regions’ importance to global weathering rates and climate. Grasslands, tundra, deserts and woodlands also experience sizable rates of rock nitrogen weathering.

Geology and carbon sequestration

Mapping nutrient profiles in rocks to their potential for carbon uptake could help drive conservation considerations. Areas with higher levels of rock nitrogen weathering may be able to sequester more carbon.

“Geology might have a huge control over which systems can take up carbon dioxide and which ones don’t,” Houlton said. “When thinking about carbon sequestration, the geology of the planet can help guide our decisions about what we’re conserving.”

Mysterious gap

The work also elucidates the “case of the missing nitrogen.” For decades, scientists have recognized that more nitrogen accumulates in soils and plants than can be explained by the atmosphere alone, but they could not pinpoint what was missing.

“We show that the paradox of nitrogen is written in stone,” said co-leading author Scott Morford, a UC Davis graduate student at the time of the study. “There’s enough nitrogen in the rocks, and it breaks down fast enough to explain the cases where there has been this mysterious gap.”

In previous work, the research team analyzed samples of ancient rock collected from the Klamath Mountains of Northern California to find that the rocks and surrounding trees there held large amounts of nitrogen. With the current study, the authors built on that work, analyzing the planet’s nitrogen balance, geochemical proxies and building a spatial nitrogen weathering model to assess rock nitrogen availability on a global scale.

The researchers say the work does not hold immediate implications for farmers and gardeners, who greatly rely on nitrogen in natural and synthetic forms to grow food. Past work has indicated that some background nitrate in groundwater can be traced back to rock sources, but further research is needed to better understand how much.

Rewriting textbooks

“These results are going to require rewriting the textbooks,” said Kendra McLauchlan, program director in the National Science Foundation’s Division of Environmental Biology, which co-funded the research. “While there were hints that plants could use rock-derived nitrogen, this discovery shatters the paradigm that the ultimate source of available nitrogen is the atmosphere. Nitrogen is both the most important limiting nutrient on Earth and a dangerous pollutant, so it is important to understand the natural controls on its supply and demand. Humanity currently depends on atmospheric nitrogen to produce enough fertilizer to maintain world food supply. A discovery of this magnitude will open up a new era of research on this essential nutrient.”

UC Davis Professor Randy Dahlgren in the Department of Land, Air and Water Resources co-authored the study.

The study was funded by the National Science Foundation’s Division of Earth Sciences and its Division of Environmental Biology, as well as the Andrew W. Mellon Foundation.

原始論文：B. Z. Houlton, S. L. Morford, R. A. Dahlgren. Convergent evidence for widespread rock nitrogen sources in Earth’s surface environment. Science, 2018; 360 (6384): 58 DOI: 10.1126/science.aan4399

引用自：University of California - Davis. "New source of global nitrogen discovered: Rocks could hold key to carbon storage and improved global climate projections."