Carbon sequestration: seeing the forest for its trees

December 18, 2000

One of the most contentious debates during the recent climate talks centered on the possible use of forests as credit towards reducing atmospheric carbon dioxide. Although it has long been assumed that these areas will act as sinks for excess carbon, the effects of species composition on the process of carbon sequestration is still largely unknown. A team of researchers working on eucalyptus plantations in Hawaii has discovered an important aspect of how carbon sequestration processes work in tropical tree plantations. The researchers, who have published their findings in the December edition of Ecology (Vol. 81, No. 12), discovered that carbon sequestration was significantly boosted when the composition of tree stands included nitrogen-fixing trees.

Jason Kaye and his colleagues from Colorado State University researched carbon storage on a former sugar cane farm which had been turned into a plantation for Eucalyptus trees (Eucalyptus saligna) in Hawaii. The team discovered that the acres which were interplanted with albizia trees (Albizia falcataria) were able to sequester more carbon than areas where eucalyptus trees were planted alone.

The researchers believe that this is due to the nitrogen-fixing qualities of the albizia trees. Albizias, which are sometimes also referred to as mimosa trees, are not a cash crop like eucalyptus. Although many tree plantations employ monoculture planting schemes, planting one tree species over hundreds of acres, the albizia trees' effect on soil quality has prompted some farmers to test the potential benefits of interplanting the two species. This kind of practice is increasingly common in tropical areas, where the nutrient level of soil is often limited.

Kaye and his colleagues studied carbon storage in forest stands planted 17 years ago with differing species composition. Some stands were planted as pure eucalyptus, some as pure albizia, and some with the two trees planted together. The researchers found that in stands where the two species were interplanted, the forest contained twice as much carbon in trees as monocrop areas. In addition, areas of pure albizia sequestered about 20 percent more carbon in soil than did the pure eucalyptus stands.

The researchers suggest that nitrogen which is added to the soil by the albizias may be inhibiting the decomposition of old, sugarcane derived, soil carbon relative to eucalyptus. Many previous studies have demonstrated that nitrogen additions tend to inhibit humus decomposition in laboratory experiments. This study, however, is the first to examine the possibility that nitrogen additions may decrease carbon turnover in the field.

The ramifications of the findings could have a large impact on the way in which the carbon sequestering potential of tropical tree plantations is measured. The global coverage of tropical tree plantations has increased dramatically in the past two decades. In 1980 it was estimated that some 21 million hectares of tropical land were being used for tree plantations globally; by 1999 estimates stood at 60 million.

In addition, nitrogen deposition has become an increasing concern worldwide, and so any information indicating that nitrogen deposition may aid in carbon sequestration may prove to be especially relevant to policymakers such as those who were in attendance at the recent Hague meetings.

"Carbon sequestration is the balance of inputs and outputs from a system," explains Kaye. "What we've shown here is that carbon outputs from soil are lower in stands that have more nitrogen-fixing trees. If decomposition is inhibited because of nitrogen inputs, then increased biological nitrogen fixation, nitrogen fertilization and nitrogen deposition may promote carbon sequestration."

The mechanisms at work in those processes, however, are still very poorly understood.

"Changes in the composition of tree species which result from land use or climate changes may have important feedbacks to terrestrial carbon sequestration," Kaye says. "We still need to learn more about how species composition may be affecting the soil of these and other forests in order to fully understand their ability to act as carbon sinks."

This study is part of an ongoing project funded by the National Science Foundation through the University of Hawaii at Hilo. The research team will follow the legacy of these species on the fertility of soils in the next generation of trees.
-end-
**Photos are available for this story. For more information please contact Alison Gillespie.**

Ecology is a peer-reviewed journal published eight times a year by the Ecological Society of America (ESA). Copies of the above article are available free of charge to the press through the Society's Public Affairs Office. Members of the press may also obtain copies of ESA's entire family of publications, which includes Ecology, Ecological Applications, and Ecological Monographs. Others interested in copies of articles should contact the Reprint Department at the address in the masthead.

Founded in 1915, the Ecological Society of America (ESA) is a scientific, non-profit, organization with over 7000 members. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. For more information about the Society and its activities, access ESA's web site at: http://esa.sdsc.edu.

Ecological Society of America

Related Nitrogen Articles from Brightsurf:

Chemistry: How nitrogen is transferred by a catalyst
Catalysts with a metal-nitrogen bond can transfer nitrogen to organic molecules.

Illinois research links soil nitrogen levels to corn yield and nitrogen losses
What exactly is the relationship between soil nitrogen, corn yield, and nitrogen loss?

Reducing nitrogen with boron and beer
The industrial conversion of nitrogen to ammonium provides fertiliser for agriculture.

New nitrogen products are in the air
A nifty move with nitrogen has brought the world one step closer to creating a range of useful products -- from dyes to pharmaceuticals -- out of thin air.

'Black nitrogen'
In the periodic table of elements there is one golden rule for carbon, oxygen, and other light elements.

A deep dive into better understanding nitrogen impacts
This special issue presents a selection of 13 papers that advance our understanding of cascading consequences of reactive nitrogen species along their emission, transport, deposition, and the impacts in the atmosphere.

How does an increase in nitrogen application affect grasslands?
The 'PaNDiv' experiment, established by researchers of the University of Bern on a 3000 m2 field site, is the largest biodiversity-ecosystem functioning experiment in Switzerland and aims to better understand how increases in nitrogen affect grasslands.

Reducing reliance on nitrogen fertilizers with biological nitrogen fixation
Crop yields have increased substantially over the past decades, occurring alongside the increasing use of nitrogen fertilizer.

Flushing nitrogen from seawater-based toilets
With about half the world's population living close to the coast, using seawater to flush toilets could be possible with a salt-tolerant bacterium.

We must wake up to devastating impact of nitrogen, say scientists
More than 150 top international scientists are calling on the world to take urgent action on nitrogen pollution, to tackle the widespread harm it is causing to humans, wildlife and the planet.

Read More: Nitrogen News and Nitrogen Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.