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article imageHow tropical forests are fertilized by pollution

Smithsonian Institution scientists previously researched the ways humans disrupt the nitrogen cycle in tropical forests; new studies at two remote Panama and Thailand sites have provided evidence of long-term nitrogen pollution effects in tropical trees.
Comparing nitrogen in dried leaves collected in 1968 with nitrogen in samples from 2007, the researchers found the proportion of heavy to light isotopes of nitrogen increased, a result similar to findings from a previous experimental fertilizing of a section of forest floor; from these changes in the proportion of nitrogen isotopes they concluded air pollution has fertilized tropical forests with active nitrogen, one of the key growth nutrients, during the 40-year period between sample collections, ScienceDaily reported about the new Smithsonian Tropical Research Institute (STRI) Global Earth Observatory study that was published November 4 in the journal Science.
Pressure-cooked into existence within stars, the element nitrogen (N), normally an odorless, colorless, mostly chemically inert gas, makes up more then 75 percent of Earth's atmosphere, and is zapped by lighting into soil and converted or "fixed" by bacteria and humans into active forms, such as ammonia (NH3), that plants and animals break down and use to build proteins and life's building blocks, in a process scientists call the nitrogen cycle; but the amount of reactive nitrogen being generated has doubled due to nitrogen fixation by humans, according to the Smithsonian team.
The ratio of 14N to 15N, the light and heavy isotopes of nitrogen, would ordinarily stay the same within an ecosystem, with only about one heavy atom in every 300 nitrogen atoms, but light nitrogen leaches out as nitrate and is lost as N2 and nitrous oxide gases, gradually increasing the heavy to light nitrogen ratio (15N/14N), and tree rings and leaf samples can provide a timeline of the nitrogen content changes in trees over the last century, according to the researchers; a greater-than-expected increase in heavy over light nitrogen at the remote Panama and Thailand sites, and at an earlier experimental site in Brazil, shows that more nitrogen has been entering the system while light nitrogen losses have increased, most likely as effects on these remotest ecosystems of the increasing amounts of nitrogen being fixed by human industries.
STRI staff scientist Ben Turner explained that the team's findings have implications for forest composition and global change models:
“The most obvious is for trees in the bean family (Fabaceae), a major group in tropical forests that fix their own nitrogen in association with soil bacteria. Increased nitrogen from outside could take away their competitive advantage and make them less common, changing the composition of tree communities.”
“There are also implications for global change models, which are beginning to include nitrogen availability as a factor affecting the response of plants to increasing atmospheric carbon dioxide concentrations. Most models assume that higher nitrogen equals more plant growth, which would remove carbon from the atmosphere and offset future warming. However a challenge for the models is that there is no evidence that trees are growing faster in Panama, despite the long-term increases in nitrogen deposition and atmospheric carbon dioxide.”
Soils and plants in temperate forests in Europe and the United States have changed markedly over the most recent decades as a consequence of increased atmospheric nitrogen deposition, the researchers claimed, urging further studies to determine whether, and if so how, tropical forests will be altered by human-generated nitrogen pollution.
More about Pollution, Air pollution, pollution tropical forests, Tropical forests, pollution fertilizes
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