A new study suggests that the capacity of tropical forests to consume carbon dioxide from the atmosphere emitted by fossil fuels is largely dependent on tree diversity. Tropical forests depend on natural nitrogen fertilizer that is infused into the soil by those trees that come from the diverse legume family which includes peas and beans.

According to a study conducted by the Princeton University, a special housing arrangement involving a certain group of tree species and a carbo-loading bacteria can determine how well the tropical forests can take in carbon dioxide found in the atmosphere.

The researchers have studied the second-growth forests found in Panama which had been utilized for agricultural purposes five to 300 years ago. During the first 12 years of recovery, the presence of legume has ensured a rapid forest growth leading to a substantial carbon ‘sink’ or what can be described as ‘carbon-storage’ capacity. Tracts of land that have been used as pasture 12 years ago had already gained carbon from fully mature forests by as much as 40 percent. Moreover, legumes have contributed more than 50 percent of the needed nitrogen to make it happen.

The secret of legumes is a process called "nitrogen fixation" which is brought out by an infectious bacteria called rhizobia. This bacterium is said to dwell in small pods found inside the tree’s roots called root nodules. Being a nutrient, nitrogen is important for plant growth. However, tropical soil lacks enough nitrogen and surprisingly, doesn’t seem to be nutritious for trees.

Legumes have secretions and use them to invite the soil-living rhizobia to infect the roots. The bacteria signal back to begin the growth of nodules. The rhizobia, which thrive on carbohydrates produced by the tree through photosynthesis, go into the host plant’s root cells and convert nitrogen found in the air into its fertilizer form that is in turn needed by the plant. The excess nitrogen produced by the legume eventually starts a nitrogen cycle that helps the neighboring trees.

The study is said to be relevant as it may help improve the carbon sink predictions by integrating nitrogen fixation in the process.

The study was published in the online journal Nature.