Associated Press

Nitrogen levels in the soil, water and atmosphere have increased fivefold globally in the past 60 years – largely due to human activities in the agricultural sector, according to a new study. Too much nitrogen can affect human health, reduce biodiversity and amplify global warming.

"Just as carbon fueled the Industrial Revolution, nitrogen has fueled an Agricultural Revolution," the study authors wrote. "The use of synthetic nitrogen fertilizers and the cultivation of nitrogen-fixing crops both expanded exponentially during the last century, with most of the increase occurring after 1960."

Where it is coming from: Nitrogen fertilizers are made by converting inert nitrogen in the atmosphere to a "fixed" chemical form plants can use as a nutrient. (The industrial process mirrors a natural one done by microbes on the roots of plants themselves.) But "fixed" nitrogen run-off can pollute neighboring land and water, and the fertilizers can spur the production of the greenhouse gas nitrous oxide by the microbes on the plants' roots. Earth has never seen this much fixed nitrogen in ecosystems worldwide, the researchers said.

Open questions: The researchers acknowledged further studies are needed to understand what such an increase in the nitrogen balance means, but preliminary estimates are unsettling. Just how much fixed nitrogen is released as greenhouse gas is still unclear but the authors said the increase in nitrogen could significantly add to problems the world is already facing from carbon and methane in the atmosphere.

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