Tropical forests are important globally in removing carbon from the atmosphere. It has been assumed that the tress were the mechanism that made this work. New research from Princeton University has shed insight on the importance of bacteria that co-exist with the trees have in absorbing atmospheric carbon.
A unique housing arrangement between a specific group of tree species and a carbo-loading bacteria may determine how well tropical forests can absorb carbon dioxide from the atmosphere, according to a Princeton University-based study. The findings suggest that the role of tropical forests in offsetting the atmospheric buildup of carbon from fossil fuels depends on tree diversity, particularly in forests recovering from exploitation.
Tropical forests thrive on natural nitrogen fertilizer pumped into the soil by trees in the legume family, a diverse group that includes beans and peas, the researchers report in the journal Nature. The researchers studied second-growth forests in Panama that had been used for agriculture five to 300 years ago. The presence of legume trees ensured rapid forest growth in the first 12 years of recovery and thus a substantial carbon “sink,” or carbon-storage capacity. Tracts of land that were pasture only 12 years before had already accumulated as much as 40 percent of the carbon found in fully mature forests. Legumes contributed more than half of the nitrogen needed to make that happen, the researchers reported.
These fledgling woodlands had the capacity to store 50 metric tons of carbon per hectare (2.47 acres), which equates to roughly 185 tons of carbon dioxide, or the exhaust of some 21,285 gallons of gasoline. That much fuel would take the average car in the United States more than half a million miles. Though the legumes’ nitrogen fertilizer output waned in later years, the species nonetheless took up carbon at rates that were up to nine times faster than non-legume trees.
The legumes’ secret is a process known as nitrogen fixation, carried out in concert with infectious bacteria known as rhizobia, which dwell in little pods inside the tree’s roots known as root nodules. As a nutrient, nitrogen is essential for plant growth, but tropical soil is short on nitrogen and surprisingly non-nutritious for trees. Legumes use secretions to invite rhizobia living in the soil to infect their roots, and the bacteria signal back to initiate nodule growth. The rhizobia move into the root cells of the host plant and — in exchange for carbohydrates the tree produces by photosynthesis — convert nitrogen in the air into the fertilizer form that plants need. Excess nitrogen from the legume eventually creates a nitrogen cycle that benefits neighboring trees.
Brown Mang Onwuka