Tropical plants, long considered a major counterforce to human-induced increase in global carbon dioxide, may not yield such a big net reduction effect after all.

Tropical plants (and plants in general) absorb carbon dioxide and water to produce sugar and oxygen. But a new study from the University of Cambridge asserts that dead tropical plant materials also trigger the release of massive amounts of carbon from soil.

The study was published online in scientific journal "Nature Climate Change" and was based on a six-year experiment in a rain forest in Panama. 

The study found that dead tropical plant materials, or litterfall, prime micro-organisms in the soil with fresh energy.  These micro-organisms are then able to decompose older organic materials in the soil and release carbon in the process.

"Older, relatively stable soil carbon is being replaced by fresh carbon from dead plant matter, which is easily decomposed" by the soil micro-organisms, said a co-author, Dr. Edmund Tanner. 

Previously, soil was thought to be simply a long-term store of carbon.

The effect of this litterfall-induced soil carbon release is so great - the study estimated that a 30 percent increase in litterfall could release 0.6 tons of carbon per hectare from lowland tropical forest soils each year - that "a large proportion of the carbon sequestered by greater tree growth in tropical forests could be lost from the soil," Cambridge reported.

"We still don't know what consequences this will have for carbon cycling in the long term," said Tanner.

Cambridge said the priming effect of tropical litterfall could affect the entire global carbon balance given the "the vast land surface area covered by tropical forests and the large amount of carbon stored in the soil."