遭受擾動的熱帶土壤是二氧化碳的潛在來源
By Zachary Boehm
加速熱帶地區的森林砍伐並增加農業土地利用，會數千年前的土壤露出地表。根據美國佛羅里達州立大學進行的新研究，這可能會把二氧化碳釋放到大氣當中。
剛果民主共和國許多曾是未受開發的土地現在則因森林砍伐而裸露。隨著該國人口持續大量增加，人們還會需要更多土地來作為農業用途。
研究人員在剛果民主共和國東部的19個地點進行研究之後，發現相較於森林密布的地區，遭到重度砍伐的區域釋放出來的有機碳年代更加古老，而且更容易受到生物分解。
這些年代較老且容易發生化學反應的碳原本位在深部土層，它們受到雨水沖刷而進入水系，最終被溪流裡的微生物用掉。微生物大量消耗這些化合物之後會把二氧化碳釋回大氣。研究人員表示此過程會危害當地的生態系，並且加重溫室效應。
「從許多方面來看，這裡正在發生的事物和一百年前的密西西比河盆地十分相似，年代更近的類比則是亞馬遜河。」研究作者，佛羅里達州立大學地球、海洋與大氣科學系的副教授Rob
Spencer表示。「剛果的原始地貌正在轉變成農地，我們想要知道這對碳循環有什麼樣的影響。」
科學家已經熟知森林砍伐在較為廣泛的層面上對碳循環的影響，而研究人員表示他們最近發表在期刊《自然―地球科學》(Nature
Geoscience)的這項發現，則提出了另外一種途徑，顯示土壤因為森林砍伐和土地利用類型轉變而受到擾動之後，其中的碳如何滲漏到河流當中。
「目前還很難得知經由此作用流失的碳有多少，因此也不容易跟其他人為的二氧化碳來源比較出它的重要性，但隨著森林砍伐和土地利用類型的轉換越趨劇烈，透過此作用排放出的二氧化碳可能也會跟著增長。」研究主要作者，在佛羅里達州立大學獲得博士學位的Travis
Drake表示。「我們希望這篇論文可以激發更多研究來探討此作用的重要性。」
在此研究中，研究人員為了把不同的土壤區分得更加明確，他們從流經研究地點的溪流中，分析溶解於水裡的有機碳。佛羅里達州立大學是國家強磁場實驗室的總部，研究人員運用此處的尖端儀器得到解析度極高的質譜儀分析數據，結果發現河流經過森林遭到砍伐的地區後，相較於經過保存良好的森林，溶解在水中的古老有機碳含有的能量更高而且化學組成更加多樣。
總體來說，森林覆蓋的地區釋放到水中的溶解性有機碳比遭到砍伐的地區多出許多。但是從遭到砍伐以及經過整地的區域中產生的溶解性有機碳卻非常「生物不穩定」(biolabile)，也就是適合給微生物利用。
「從成份上來看，遭到砍伐的地貌中釋放出來的溶解性有機碳，充滿了微生物偏好的種類――它們的成份較為簡單且容易利用，還含有許多氮。」現於蘇黎世聯邦理工學院進行研究的Drake表示。「觀察森林遭到砍伐地區的河流，可以發現其中的二氧化碳濃度較高。我們認為一部分的原因，便是微生物消耗了從土壤中釋放出來的古老有機碳。」
在剛果這類開發中的熱帶地區，森林砍伐造成的土壤擾動，可能會經由降雨而大量增加溶出的有機碳。有機物流失會降低土壤的沃度。而且可以支持水生和海岸生態系的重要營養物質，運送到下游的量也會變少。
從更加廣大的層面來看，這個過程代表了嚴密封存在地下數千年的碳，可以重新進入到現在的碳循環。如果這些碳真如研究人員斷定的會成為二氧化碳進入大氣之中，溫室效應便會因此加劇。
砍伐森林、土地利用轉型以及其他還沒調查的過程都會擾動熱帶地區富含養分的土壤，研究人員表示研究結果強調出我們急需找出這類過程產生的二階與三階效應。雖然有系統地擴大森林保護的面積是最佳良藥，但論文建議使用干擾程度較低的農耕方法，也有助於抵消農耕造成的土壤擾動。
Spencer說：「這項研究的重點放在剛果，因為農業造成的土地利用類型轉變，目前來說規模最大的地區正是熱帶。」
「要把碳長時間封存在土壤當中，保護森林到頭來還是最有用的方法。」Drake接著表示，「但土地利用類型非得轉變時，採用較好的方法，像是用梯田來耕種、配置植生緩衝帶、使用有機堆肥等，都可以改善我們觀察到有機碳被沖刷出來的現象。」
Tropical soil disturbance could be
hidden source of CO2
Thousand-year-old tropical soil unearthed
by accelerating deforestation and agriculture land use could be unleashing
carbon dioxide into the atmosphere, according to a new study from researchers
at Florida State University.
In an investigation of 19 sites in the eastern
Democratic Republic of the Congo, researchers discovered that heavily
deforested areas leach organic carbon that is significantly older and more
biodegradable than the organic carbon leached from densely forested regions.
Released from deeper soil horizons and leached by
rain into waterways, that older, chemically unstable organic carbon is
eventually consumed by stream-dwelling microbes, which devour the rich
compounds and respire carbon dioxide back into the atmosphere. It’s a process
that could jeopardize local ecosystems and further fuel the greenhouse effect,
researchers said.
“In many ways, this is similar to what happened in
the Mississippi River Basin 100 years ago, and in the Amazon more recently,”
said study author Rob Spencer, an associate professor in FSU’s Department of
Earth, Ocean and Atmospheric Science. “The Congo is now facing conversion of
pristine lands for agriculture. We want to know what that could mean for the
carbon cycle.”
While the broader effects of deforestation on the
carbon cycle are well known, researchers said their findings, published today
in the journal Nature Geoscience,
suggest there is an additional pathway or leakage of carbon into rivers from
soil churned by deforestation and land conversion.
“At this point, it’s hard to know the magnitude of
this flux and thus the relative importance of this process compared to other
anthropogenic sources of CO2, but it is likely to grow with
additional deforestation and land-use conversion,” said former FSU doctoral
student Travis Drake, the study’s lead author. “We hope this paper stimulates
more research into the relative importance of this process.”
To better distinguish the different soils in their
study, researchers analyzed the dissolved organic carbon drained from study
sites into outflowing streams and rivers. Using ultrahigh-resolution mass
spectrometry data generated by cutting-edge tools at the FSU-headquartered
National High Magnetic Field Laboratory, the team found that the older
dissolved organics discharged from deforested areas were more energy-rich and
chemically diverse than those from better-preserved forests.
Overall, forested areas released significantly more
dissolved organic carbon than deforested areas. But the dissolved organics that
did emanate from the deforested and land-converted regions were exceptionally
biolabile, or suited for microbial consumption.
“Compositionally, the dissolved organics from
deforested landscapes were full of the kinds of things microbes prefer to eat —
simpler and easily accessible compounds with plenty of nitrogen,” said Drake,
who now conducts research at the Swiss Federal Institute of Technology in
Zürich. “We think the microbial consumption of these old organics coming from
soils may partially explain the higher concentrations of CO2 we observed in the
deforested area streams.”
In developing tropical regions like the Congo,
deforestation-related soil disturbance has the potential to dramatically
increase leaching of organic carbon by rainfall. That loss of organic matter
could compromise soil fertility and reduce the downstream transport of critical
nutrients that support aquatic and coastal ecosystems.
More broadly, this process means carbon that was
safely sequestered in the Earth for millennia could now be re-entering the
modern carbon cycle. If, as researchers posit, that carbon is eventually
released into the atmosphere as carbon dioxide, it could contribute to the
greenhouse effect.
Researchers said these findings underscore the
urgency of identifying the second- and third-order effects of deforestation,
land conversion and the unchecked disturbance of deep, nutrient-rich soils in
the tropics. While widespread and systematic forest preservation is the best
antidote, the paper suggests less disruptive farming practices could help
offset some of the destabilization.
“This research focuses on the Congo because the
tropics are really at the forefront of agriculture-driven land-use conversion,”
Spencer said.
“Ultimately, it depends on the preservation of
forests that maintain and store carbon in soils over longer timescales,” Drake added.
“When land-use conversion does occur, better practices such as terracing, use
of buffer strips and application of organic residues could ameliorate some of
the observed organic carbon leaching.”
原始論文：Travis W.
Drake, Kristof Van Oost, Matti Barthel, Marijn Bauters, Alison M. Hoyt, David
C. Podgorski, Johan Six, Pascal Boeckx, Susan E. Trumbore, Landry Cizungu
Ntaboba, Robert G. M. Spencer. Mobilization of aged and biolabile soil
carbon by tropical deforestation. Nature Geoscience, 2019;
DOI: 10.1038/s41561-019-0384-9
引用自：Florida State University. "Tropical soil
disturbance could be hidden source of CO2."
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