Nitrogen is colorless, odorless and tasteless, but all life on earth depends on it. Without it, our bodies cannot synthesize the nucleic acids that make up our DNA, or the protein-forming amino acids that are the very building blocks of our cells. Problematically, atmospheric nitrogen is relatively inert or nonreactive.
This has created a unique biological dependency on a process called nitrogen fixation—where inert nitrogen from the atmosphere is converted into more reactive ammonia, a major component of soil fertilizers. A recent discovery has revealed just how reliant recovering forests are on nitrogen fixation, and how some forests can even manipulate it to speed up their abilities to ‘heal’ themselves. Many tropical forests today have been exploited for agriculture, mining, fossil fuel exploitation, and other human use. “It is therefore crucial to resolve whether nitrogen-fixing trees can relieve nitrogen limitation and ensure fixation as an ecosystem service during tropical forest secondary succession,” writes Sarah Batterman of Princeton University in a recent article published in the journal Nature. A coalition of researchers from several institutions—Princeton University, the Smithsonian Tropical Research Institute, Wageningen University in the Netherlands, the University of Copenhagen in Denmark, Yale University, and the German Center for Integrative Biodiversity Research—set out to investigate the role played by nitrogen-fixing trees in tropical forest recovery. They chose to study forest plots in Panama, under the auspices of the Agua Salud Project that oversees the management of the watershed that supplies the Panama Canal.
First, they estimated the biomass of the trees in a forest plot and the proportion of each plant’s biomass that was present in its roots, leaves or stems. Using known root and stem (or wood) nitrogen levels, they calculated how much of the tree’s biomass consisted of nitrogen. In the case of leaves, which are the most individually variable components of any plant, coauthor Dylan Craven of Yale University assessed the average nitrogen levels present in the leaves of over fifty species of tropical Panamanian plants.
Subsequently, the researchers also measured nitrogen fixation in individual trees in each forest plot by counting the number of leguminous nodules present on tree roots, which have a symbiotic relationship with nitrogen-fixing bacteria. They then measured how quickly the bacteria in these nodules could fix nitrogen.
Armed with these data, the scientists compared plots that had been abandoned from pasture five, twelve, thirty, and eighty years ago, against 300-year-old mature forests.
Brown Mang Onwuka