What if tropical forest soils start releasing more carbon as global warming increases?

As the amount of CO2 in the atmosphere increases trees in tropical forests grow faster and more carbon is stored above ground in the increased forest biomass. But a paper recently published in Nature Climate Change suggests that we should take a somewhat less optimistic view of how much carbon might stored in forests as the climate warms.

The research, published under the title, “Soil carbon release enhanced by increased tropical forest litterfall“, was led by scientists from the Centre for Ecology and Hydrology and the University of Cambridge, UK. It is based on six years of research in a lowland rainforest at the Smithsonian Tropical Research Institute in Panama.

The findings may seem counterintuitive at first. As CO2 levels in the atmosphere rise, the net primary productivity in tropical forests increases. Along with greater growth of trees and bushes, comes more litter production and more organic matter on the forest floor. We might expect that this would mean more carbon stored in the soil. But increased litterfall (dead leaves, bark and twigs that fall to the ground) can increase the carbon released from the soil. The reason is the “priming effect” of easily decomposable organic matter, which stimulates microbial activity and results in an increase of CO2 emissions from the soil.

The scientists conclude that a large proportion of the carbon sequestered by greater tree growth in tropical forests as a result of more CO2 in the atmosphere could be lost from the soil.

We predict that a future increase in litterfall of 30% with an increase in atmospheric CO2 concentrations of 150 ppm [parts per million] could release about 0.6 tC/ha/year from the soil, partially offsetting predicted net gains in carbon storage.

The authors point out that their estimate of 0.6 tC/ha/year is higher than previous estimates of increase in forest biomass in the Amazon rainforest as a result of increased CO2 in the atmosphere (0.45 t/ha/yr).

In the supplementary information to the paper, the authors explain that an increase of 150 parts per million could happen within 50-100 years and that their estimate of 0.6 tC/ha/year might be on the conservative side. In addition, priming effects may be even greater because root biomass and root exudations (emissions) are anticipated to increase with increased CO2. Thus further increases the rate of carbon release due to priming.

In their study, the authors explain that increased litterfall could reduce the stability of soil organic carbon in the long run, because stable soil carbon would be replaced with carbon that is more susceptible to microbial decomposition.

In a press release, co-author Dr Edmund Tanner from the University of Cambridge, said,

“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.”

The researchers consider the impact that drought might have on tropical forests in their paper. By restricting microbial decomposition, droughts could reduce carbon release from the soil. But since trees respond to drought stress by losing leaves resulting in increased organic matter on the ground, this could result in enhanced priming effects as soon as the drought ends and decomposition starts again.

Lead author Dr Emma Sayer from the Centre for Ecology and Hydrology in the UK said in a press release,

“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.”

None of this is to argue against saving tropical forests from the chainsaw. But measuring the amount of carbon stored in tropical forests is an extremely complex business. Sayer et al’s research reveals how complex measuring the carbon stored in forests and forest soils actually is. As the researchers write,

it is essential that plant–soil feedbacks are taken into account in predictions of the carbon sequestration potential of tropical forests.

It seems like a reasonable suggestion. Although the authors do not point this out, it is also an excellent argument against trading the carbon stored in tropical forests, since we currently do not know how much carbon is stored in forest soils, or how much will remain stored under a changing climate.