America's national parks receive harmful, accidental fertilization from air pollution. A recent study identified how this unwanted dose of nitrogen nutrients harms plants and disrupts aquatic environments in 38 U.S. national parks.
Much of this accidental fertilization comes from ammonia that evaporated from agricultural chemicals and livestock urine, according to the study published in Atmospheric Chemistry and Physics. Power plant and vehicle emissions also contribute nitrogen in the form of nitrous oxides.
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These nitrogen-containing gases enter the atmosphere, then fall back to earth in rain water or through other means.
"When we apply fertilizer in the United States, only about 10 percent of the nitrogen makes it into the food," said lead author Daniel J. Jacob of Harvard University in a press release. "All the rest escapes, and most of it escapes through the atmosphere."
Once that nitrogen escapes into the environment, it damages ecosystems.
In the east, the unwanted fertilizer most seriously affects the hardwood trees, such as oak, in Great Smoky Mountains National Park. The trees start to suffer when nitrogen input reaches 3 to 8 kilograms per hectare each year. However, the forest now receives far more than that, at 13.6 kilograms per hectare each year.