A new study indicates that forests, prairies and other natural areas around the globe acquire less nitrogen than previously estimated.
The findings have climate implications as plants need the element to remove carbon dioxide from the atmosphere.
Published today in Nature, the data analysis of biological nitrogen fixation also shows a rise in agricultural nitrogen fixation that may be contributing to the degradation of land, air and water quality.
Researchers led by Carla Reis Ely, a postdoctoral scholar in the Oregon State University College of Forestry during the study, found that estimates of nitrogen fixation had been skewed by sampling bias: Field measurements of nitrogen fixation in natural areas had been taken in places where nitrogen-fixing organisms were 17 times more prevalent than they are worldwide.
Nitrogen makes up more than three-quarters of the Earth's atmosphere (most of the rest is oxygen) and is essential to a range of life systems, even though most organisms are unable to use it directly.
Nitrogen-fixing bacteria convert nitrogen gas, N2, in the air into forms such as ammonia that plants can use to grow, stay healthy and reproduce. They need nitrogen to make proteins and chlorophyll, the green pigment in leaves that enables photosynthesis, through which they use atmospheric carbon dioxide to produce their food.
"In natural ecosystems, nitrogen fixation improves soil fertility and supports plant growth, thereby increasing carbon storage," said Reis Ely, who led an international team of 24 scientists on the study. "However, our new estimate of natural nitrogen fixation, based on improved scientific understanding, suggests less new nitrogen enters natural ecosystems."
One implication of this finding is that these ecosystems may take up less carbon dioxide than scientists had thought previously, she said. That means natural ecosystems could have a lower capacity to store carbon and mitigate climate change.
Meanwhile, the rise in agricultural nitrogen fixation, through the planting of legumes such as soybeans and alfalfa that host nitrogen-fixing bacteria, is both a positive and a negative, she said. Nitrogen fixation is essential for producing food for a growing global population and is generally more environmentally sustainable than synthetic nitrogen fertilizers, but food waste in the production and consumption chain is a big contributor to nitrogen pollution.
"Using nitrogen-fixing crops in rotation can support long-term soil health and reduce environmental damage from chemical fertilizers," she said. "But too much nitrogen can throw off the overall balance of nutrients in the soil, and excess nitrogen can leach into groundwater or run off into lakes and streams, causing algae blooms and harming aquatic life."
In addition, surplus nitrogen can become nitrous oxide, a potent greenhouse gas, and high nitrogen levels can favor fast-growing invasive plants that squeeze out native species and reduce biodiversity.
"High agricultural nitrogen fixation is a complex issue," said Reis Ely, now a scientist with the Oak Ridge Institute for Science and Education. "We do benefit from it, but along with applications of synthetic nitrogen fertilizer, it contributes to nitrogen pollution and climate change. Measuring and monitoring biological nitrogen fixation need to be ongoing to help us ensure we have all the nitrogen in the ground that we need without it becoming too much of a good thing."
