The theory sinks

Two new studies quash industrialised countries hopes of meeting most of their Kyoto commitments by using carbon sinks

Forests and soil 'sinks' may not eventually turn out to be so useful to industrialised countries in search of cheap ways to reduce greenhouse gas emissions. Two scientific studies conducted in North American pine forests present evidence that estimates of increased absorption of carbon dioxide as its concentration in the atmosphere rises are unduly optimistic.

It is widely understood, though voices of dissent have been expressed, that with increasing levels of carbon dioxide, plants will absorb more of it through photosynthesis. This process is termed as carbon dioxide fertilisation. The authors of the two studies, carried out at the scale of a forest stand in the Duke Forest, North Carolina, US have, however, proved otherwise.

The first study demonstrates that soil fertility can limit the amount of carbon dioxide absorbed by trees, no matter how enriched the air is in terms of its carbon dioxide concentration. The second study conducted on the same site suggests that forest soils are unlikely to play a significant role, as a reservoir of carbon dioxide, in retaining the gas for long periods. The studies do not bring good news for industrialised countries like Canada and Japan keen on using forests and soils to remove carbon dioxide from the atmosphere, instead of cutting fossil fuel use.

In 1994, an experimental area in a 11-year old, loblolly pine (Pinus taeda L.) plantation was fumigated with carbon dioxide to increase concentration levels to 550 parts per million by volume (ppmv). Present atmospheric concentrations of carbon dioxide are about 365 ppmv. In the initial three years, trees in the experimental plot had an average 34 per cent increase in carbon dioxide absorbed relative to an adjacent untreated plot, which was not fumigated. But this increase declined to six per cent over the next four years.

The reason for an initial high, but transient, increase in absorption was explained when a balanced fertiliser was added to one half of each plot. Over a period of two years, a remarkable 47 per cent increase in annual growth of trees under enhanced carbon dioxide and improved nutrition, due to addition of fertiliser, was observed. On the other hand, trees in fumigated areas where no nutrient was added showed a small increase of 7 per cent. A slightly higher increase of 15 per cent was seen in areas with improved nutrient level but no elevated carbon dioxide. This shows a synergy between improved nutrition and enhanced carbon dioxide concentration, which can substantially raise annual tree growth, and consequently, the amount of carbon dioxide absorbed.

This synergy becomes stronger as soil fertility worsens. At one such site with infertile sandy soil, improved nutrition and enhanced carbon dioxide caused a 74 per cent increase in annual tree growth compared to a 47 per cent increase in moderately fertile soil.

If present trends continue, an extra 4.3 kilogramme of carbon per square metre (kg C/m2) would be sequestered after 40 years owing to carbon dioxide fertilisation. However, if lack of nutrients limit increase in growth to only 3 years, this extra carbon sequestration will cease in at most ten years at a cumulative amount of less than 1kg C/m2.

In the second study, initiated in 1996, soil samples collected after three years of tree growth at elevated carbon dioxide levels did not show any accumulation of carbon in the deeper layers of mineral soil. Stored carbon stays longer in deeper layers at a depth ranging from 15 - 30 cm. On the other hand, the litter layer - layer of dead plant material on top of the mineral soil - showed high accumulation. Carbon stored in this layer is not very stable and can be released back to the atmosphere in just about three years. "Such findings call into question the role of soils as long-term carbon sinks, and show the need for a better understanding of carbon cycling in forest soils," say authors of the study, William H. Schlesinger, Duke University and John Lichter, Bowdoin College, Maine.

These studies have elicited a mixed reaction from the scientific community. According to some, this information can help in improving models to predict the impact of rising concentrations of greenhouse gas on our ecosystems and climate. "Assuming that sinks will be there forever will send us down the wrong policy path," said Jim Ehleringer, a biologist at University of Utah. Others, exercising caution, call the results as preliminary since forests can take a long time to adjust to changes in the environment.