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Study finds enhanced tropical forest growth could result in carbon release from soil; “priming” stimulates decomposition

A new study led by scientists from the Centre for Ecology & Hydrology and the University of Cambridge, UK shows that as climate change enhances tree growth in tropical forests, the resulting increase in litterfall could stimulate soil micro-organisms, leading to a release of stored soil carbon. The results were published in the journal Nature Climate Change.

The researchers used results from a six-year experiment in a rainforest at the Smithsonian Tropical Research Institute in Panama, Central America, to study how increases in litterfall—dead plant material such as leaves, bark and twigs which fall to the ground—might affect carbon storage in the soil. Their results show that extra litterfall triggers an effect called “priming” in which fresh carbon from plant litter provides much-needed energy to micro-organisms, which then stimulates the decomposition of carbon stored in the soil.

Most estimates of the carbon sequestration capacity of tropical forests are based on measurements of tree growth. Our study demonstrates that interactions between plants and soil can have a massive impact on carbon cycling. Models of climate change must take these feedbacks into account to predict future atmospheric carbon dioxide levels.

—Dr. Emma Sayer, Centre for Ecology & Hydrology

The study concludes that a large proportion of the carbon sequestered by greater tree growth in tropical forests could be lost from the soil. The researchers estimate that a 30% increase in litterfall could release about 0.6 tonnes of carbon per hectare from lowland tropical forest soils each year. This amount of carbon is greater than estimates of the climate-induced increase in forest biomass carbon in Amazonia over recent decades. Given the vast land surface area covered by tropical forests and the large amount of carbon stored in the soil, this could affect the global carbon balance.

Tropical forests play an essential role in regulating the global carbon balance. Human activities have caused carbon dioxide levels to rise but it was thought that trees would respond to this by increasing their growth and taking up larger amounts of carbon. However, enhanced tree growth leads to more dead plant matter, especially leaf litter, returning to the forest floor and it is unclear what effect this has on the carbon cycle.

Soils are thought to be a long-term store for carbon but we have shown that these stores could be diminished if elevated carbon dioxide levels and nitrogen deposition boost plant growth.

—Dr. Emma Sayer

This priming effect essentially means that older, relatively stable soil carbon is being replaced by fresh carbon from dead plant matter, which is easily decomposed. We still don't know what consequences this will have for carbon cycling in the long term.

—Dr Edmund Tanner, from the University of Cambridge

Resources

  • Sayer, E et al. (2011) Soil carbon release enhanced by increased tropical forest litterfall. Nature Climate Change. doi: 10.1038/nclimate1190

Comments

Reel$$

"However, enhanced tree growth leads to more dead plant matter, especially leaf litter, returning to the forest floor and it is unclear what effect this has on the carbon cycle."

http://www.youtube.com/watch?v=P2qVNK6zFgE

Of course the stem, trunk and root weight of trees/plants continue to grow and sequester ever greater amounts of carbon (143% with cowpea) .... So umm, this study tell us nothing. Except worry, worry, worry.

ToppaTom

Simple
"Tell 'em more trees = more CO2.
and less trees = more CO2.
Any problem with that?"

ai_vin

Reel and his youtube are right: More CO2 does equal more plant growth - in a controlled environment like that lab. However, outside the lab CO2 isn't the only thing that effects plant growth and plants aren't the only things that like more CO2;
http://www.youtube.com/watch?v=R1M_BfJaEBM

http://www.youtube.com/watch?v=g093lhtpEFo

Reel$$

The first video was amusing ai since the "researchers" somehow convinced themselves that piping CO2 outdoors makes a controlled experiment.

The second video is about flooding in Pakistan - having nothing to do with CO2's fertilization effect.

The video I provide shows results of control increased atmospheric CO2 on plant stem and root growth. Far more germane.

ai_vin

First off, the second vid is about a lot more than flooding in Pakistan.

And second, if you think the researchers in the first vid just released CO2 into the air and didn't also measure and test for where it went...well you don't know how science works.

As for you vid, it only mentioned the positives like growth. I've seen that experiment done and there are some negatives, even in the lab. With more CO2 the water use of the plant changes, the nutrient content(stuff other than carbos) is often less, only SOME plants actually do grow better (weeds do and they compete with crops), etc.

ai_vin

http://skepticalscience.com/co2-is-plant-food-too-simple.html

http://blogs.edf.org/climate411/2007/08/22/beyond-climate-carbon-and-cows/

Reel$$

The 3rd link you offer is better in that at least it uses the Brookhaven FACE studies where the CO2 fertilization is described:

Plants respond to rising [CO2] through increased photosynthesis and reduced transpiration. Photosynthesis removes CO2 from the atmosphere and respiration by plants and heterotrophs add it back.

The conclusion is telling:

"The global increase of CO2 is thus a grand biological experiment, with countless complications that make the net effect of this increase very difficult to predict with any appreciable level of detail."

Keep in mind paleobiology tells us the Devonian Period with 900-1200ppm CO2 hosted Earth's flourish of first forests and seed plants.

The 4th link is an interpretation of a Nature study about prairie grasses nutrition value to beef cattle. Most beef cattle are corn fed today. And should cattle go back to pasture graze grass they will be far less stressed than in feed lots. So they eat more.

ai_vin

Keep in mind the plants and "forests" of the Devonian were unlike what we have now and we don't know if they used the C3 or C4 carbon pathways.

Early Devonian plants did not have roots or leaves like the plants most common today and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. By far the greatest land organism was Prototaxites, the fruiting body of an enormous fungus that stood more than 8 meters tall, towering over the low, carpet-like vegetation. By Middle Devonian, shrub-like forests of primitive plants existed: lycophytes(club moss), horsetails, ferns, and progymnosperms had evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest known trees, from the genus Wattieza [Wattieza had fronds rather than leaves, were related to the ferns and horsetails, and reproduced with spores.], appeared in the Late Devonian around 380 Ma. In the Late Devonian, the tree-like ancestral fern Archaeopteris and the giant cladoxylopsid trees grew with true wood. These are the oldest known trees of the world's first forests. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion".

The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event. See Late Devonian extinction.

It was a different world, the sun was cooler, the continents were arranged differently, and sea levels were higher because there were no large ice sheets covering the land.

http://www.devoniantimes.org/opportunity/massExtinction.html

Reel$$

Interesting link ai - thanks. Significantly the extinctions occurred in the oceans. Marine life suffered, not terrestrial plant life.

A variety of causes have been proposed for the Devonian mass extinctions. These include asteroid impacts, global anoxia (widespread dissolved oxygen shortages), plate tectonics, sea level changes and climatic change.

"Devonian Plant Hypthesis:" Expanding forests creating huge carbon sinks flushing organic nutrients to the oceans may have caused eutrophication in shallow seas and subsequent extinctions.

It would be interesting if human forest mitigation in CO2 enriched atmosphere somehow contains eutrophication in the modern era.

"Ironically, the development and maturation of terrestrial environments fostered by the expansion of terrestrial plants may have wrecked havoc on the oceans from which life first arose."

ai_vin

Yeah, and it wasn't just that the plants themselves were carbon sinks. This was the time when plants first evolved root systems. These tall trees required deep rooting systems to acquire water and nutrients, and provide anchorage. These systems broke up the upper layers of bedrock and stabilised a deep layer of soil, which would have been on the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a couple of centimetres. The mobilisation of a large portion of soil had a huge effect; soil promotes weathering, the chemical breakdown of rocks, releasing ions which act as nutrients to plants and algae. The weathering of silicate rocks also draws down carbon dioxide from the atmosphere.

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