古馬雅的伐林活動至今仍影響著中美洲土壤的碳貯存量
這項發現強調出土壤管理對未來溫室氣體含量的潛在影響
有些人猜測砍伐森林造成馬雅文化在一千多年前神秘地覆亡。一項新研究顯示猶加敦半島上熱帶土壤的碳儲庫同樣因為除林活動而大幅削減，即使在這些古代城市遭到遺棄而森林重新回歸了很長一段時間之後。
這項發現發表於期刊《自然―地質科學》(Nature Geoscience)，顯示在決定地球未來大氣中的溫室氣體含量時，土壤本身以及我們對待它的方式扮演了相當重要的腳色。
過往研究指出馬雅人大約是在距今4000年前開始進行農耕，而農業擴展與建造城市最終導致了大範圍的伐林活動與土壤侵蝕。這篇新論文的主要作者，麥基爾大學的地球化學家Peter Douglas表示此篇新研究最令人驚訝的發現是經過一千多年的林地復育之後，該區域土壤吸收碳的能力仍然還沒完全復原。
生態系的「基礎」已經改變
「如果你今日前往那個地區，大部分的景色看起來都是濃密的原生雨林。」麥基爾大學地球與行星科學的助理教授Douglas表示，「但觀察土壤碳儲庫，則當地的生態系已經發生了本質上的改變，再也無法回歸原有的狀態。」
土壤是地球上儲存最多碳的地方之一，其含有的碳至少是現今大氣中的碳的兩倍。然而，在比十年還長的時間尺度下，科學家對於土壤的碳儲庫會如何變化仍所知甚少。這篇新研究連同最近發表的其他研究，提出土壤碳儲庫在跨越數個世紀甚至數千年的時間尺度下會有驚人的變化發生。
為了研究這方面的長期效應，Douglas和共同作者探討從三座湖泊底部取出的沉積物岩芯。這些湖泊位於墨西哥南部和瓜地馬拉的馬雅低地。研究人員測量會隨著時間衰變的放射性碳同位素，來得到植物蠟分子的年代。由於植物蠟在土壤裡面會和礦物結合，因此通常可以在土壤中保存很長一段時間。接著，他們比較植物蠟分子的年代以及和沉積物一起沉積的植物化石年代。
研究團隊包括耶魯大學、蘇黎世聯邦理工學院、佛羅里達大學和威斯康辛大學蘇必略分校的科學家，他們發現當古馬雅人開始砍伐附近的森林，湖泊沉積物中植物化石和植物蠟的年代差距就會從相當大變成相當小。這意謂著碳被保存在土壤中的時間縮短了許多。
這項計畫源自於Douglas數年前在耶魯大學攻讀博士時進行的研究，當時他利用植物蠟分子來追蹤過往的氣候變化對古馬雅有何影響。同一時間，其他研究人員進行的研究則指出植物蠟分子也是了解土壤碳儲庫的良好示蹤劑。Douglas解釋：「將這些研究成果結合在一起，我們意識到此處是個重要的資料庫，可以找出古代人類的伐林活動和土壤碳儲庫變化之間的關聯。」
保護熱帶原生林
「除了現有的一長串名單之外，這給了我們另一個理由去保護世上剩下的熱帶原生林。」Douglas表示，「此研究在設計碳補償之類的政策時也具有其意義。碳補償通常會列入重新造林，但在長期儲碳上其實無法完全抵消。」(碳補償允許企業或個人藉由購買環境計畫的信用額度，像是造林等，來補償自身的溫室氣體排放量)
研究人員所用的技術不過是在最近才發展出來。在後續幾年「我們希望能分析世界其他地區的熱帶森林，來觀察是否有同樣的模式出現――也就是人類過去的伐林與農業活動是否至今仍影響著全球的土壤碳儲庫。」Douglas表示，「此外，我也想把這項技術應用在加拿大的永凍土區域，藉此觀察過往氣候變遷發生時永凍土中儲存的碳發生了什麼變化。」
Carbon reserves in Central
American soils still affected by ancient Mayan deforestation
Finding underscores potential impact of soil
management on future greenhouse-gas levels
Deforestation is suspected to have contributed to the
mysterious collapse of Mayan civilization more than 1,000 years ago. A new
study shows that the forest-clearing also decimated carbon reservoirs in the
tropical soils of the Yucatan peninsula region long after ancient cities were
abandoned and the forests grew back.
The findings,
published in the journal Nature
Geoscience, underscore how important soils and our treatment of
them could be in determining future levels of greenhouse gases in the planet’s
atmosphere.
The Maya began farming
around 4,000 years ago, and the spread of agriculture and building of cities
eventually led to widespread deforestation and soil erosion, previous research
has shown. What’s most surprising in the new study is that the soils in the
region haven’t fully recovered as carbon sinks in over a millennium of
reforestation, says McGill University geochemist Peter Douglas, lead author of
the new paper.
Ecosystem
‘fundamentally changed’
“When you go to this
area today, much of it looks like dense, old-growth rainforest,” says Douglas,
an Assistant Professor of Earth and Planetary Sciences at McGill. “But when you
look at soil carbon storage, it seems the ecosystem was fundamentally changed
and never returned to its original state.”
Soil is one of the
largest storehouses of carbon on Earth, containing at least twice as much
carbon as today’s atmosphere. Yet scientists have very little understanding of
how soil carbon reservoirs change on timescales longer than a decade or so. The
new study, along with other recently published research, suggests that these
reservoirs can change dramatically on timescales spanning centuries or even
millennia.
To investigate these
long-term effects, Douglas and his co-authors examined sediment cores extracted
from the bottom of three lakes in the Maya Lowlands of southern Mexico and
Guatemala. The researchers used measurements of radiocarbon, an isotope that
decays with time, to determine the age of molecules called plant waxes, which
are usually stored in soils for a long time because they become attached to
minerals. They then compared the age of wax molecules with that of plant
fossils deposited with the sediments.
The team – which
included scientists from Yale University, ETH Zurich, the University of Florida
and the University of Wisconsin-Superior – found that once the ancient Maya
began deforesting the landscape, the age difference between the fossils and the
plant waxes went from being very large to very small. This implies that carbon
was being stored in soils for much shorter periods of time.
The project stemmed
from research that Douglas had done several years ago as a PhD student at Yale,
using plant-wax molecules to trace past climate change affecting the ancient
Maya. At the same time, work by other researchers was indicating that these
molecules were a good tracer for changes in soil-carbon reservoirs. “Putting
these things together, we realized there was an important data-set here
relating ancient deforestation to changes in soil carbon reservoirs,” Douglas
explains.
Protecting
old-growth tropical forests
“This offers another
reason – adding to a long list – to protect the remaining areas of old-growth
tropical forests in the world,” Douglas says. “It could also have implications
for how we design things like carbon offsets, which often involve reforestation
but don’t fully account for the long-term storage of carbon.” (Carbon offsets
enable companies or individuals to offset their greenhouse-gas emissions by
purchasing credits from environmental projects, such as tree-planting.)
The technique used
by the researchers has been developed only recently. In the years ahead, “it
would be great to analyze tropical forests in other regions of the world to see
if the same patterns emerge — and to see if past human deforestation and
agriculture had an impact on soil carbon reservoirs globally,” Douglas says.
“I’m also very interested in applying this technique to permafrost regions in
Canada to see what happened to carbon stored in permafrost during previous
periods of climate change.”
原始論文：Peter M. J. Douglas, Mark Pagani, Timothy I.
Eglinton, Mark Brenner, Jason H. Curtis, Andy Breckenridge, Kevin
Johnston. A long-term decrease in the persistence of soil carbon caused
by ancient Maya land use. Nature Geoscience, 2018; DOI: 10.1038/s41561-018-0192-7
引用自：McGill University. "Carbon reserves in Central American
soils still affected by ancient Mayan deforestation."
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