Increasing Nitrogen in Earth's Soils May Signal Global Changes

According to new research, the rapid increase of airborne nitrogen resulting from fossil-fuel combustion and crop fertilization, combined with carbon stored in Earth's soils, may change the rate of carbon dioxide (CO2) rising into the atmosphere.

About 300 times more carbon is stored in soils than is being put in the atmosphere by humans every year in the form of C02, and each year soils release about 20 times more carbon through decomposition than industrial activities.

"Decomposition is primarily balanced by plant growth, but increasing nitrogen falling on ecosystems could change that balance," said biologist Alan Townsend of the University of Colorado at Boulder. Townsend's new NSF-supported study shows tundra soils are unexpectedly sensitive to added nitrogen. The findings suggest that we need to understand how human-caused increases in nitrogen throughout the world might affect C02 storage areas, or sinks, on land, said Townsend.

The study area of nitrogen deposition is in the tundra at Niwot Ridge, around 30 miles west of Boulder, a Long-Term Ecological Research Site funded by the National Science Foundation. The big surprise is that many scientists believed soils would not respond much to changes in nitrogen.

Researchers believe that C02 in the atmosphere has risen by about one-third since the Industrial Revolution began in roughly 1760, contributing to a warming climate.

"One of our big concerns now is that we know the world's soils have at least 3 times more carbon than plants, and that increasing the nitrogen hitting these soils could change the size of that huge pool," said Townsend. "Since the pool is so large, even a small change could have a big effect on the atmosphere, and therefore future climate," he said.

Scientists have documented increases in C02 produced by human activity, and concluded that only about half of that amount is reaching the atmosphere, said Townsend. Therefore, he added, the carbon sinks on Earth in the world's vegetation, soils and oceans must be immense.

"If these sinks slow down - or turn off - in the near future, we could see much larger increases of atmospheric C02. If cold tundra soils are sensitive to nitrogen, it raises concerns about what might be happening in other, warmer parts of the world where things can change more rapidly," said Townsend.

Niwot Ridge is by no means unique, believes Townsend. "Nitrogen deposition is going up all over the world, especially throughout the United States, Europe and much of Asia. I think the problems we are seeing from the altered nitrogen cycle are worse than what we are seeing in climate change around the world, at least for now."