Clean forests prompt pollution rethink

South American streams call current nitrogen-cycle theory into question.

Pollution may have altered northern hemisphere forests dramatically. The surprise finding that clean forests use nitrogen differently to polluted ones emphasizes the effect that humans have on the planet’s nitrogen cycle¹. It may even prompt a rethink of the way that this cycle works.

Humans have added vast amounts of nitrogen to the earth’s ecosystems. The element fertilizes plants. To understand what affect this has had already, and how the planet might fare in the future, we need to know how forests used nitrogen before this artificial influx began.

So Steven Perakis and Lars Hedin, ecologists at Cornell University, Ithaca, New York, searched the globe for an environment with as little nitrogen pollution as possible. They settled on 100 streams in temperate forests in Chile and Argentina, far from industry.

They found that more than three-quarters of the nitrogen in these streams is organic (combined with carbon). Nitrogen from pollution is overwhelmingly inorganic – chemically bound to oxygen, hydrogen or metals.

Over 70% of the nitrogen in woodland rivers in Europe and North America is in the form of inorganic nitrate. But in the 100 South American streams, nitrate was the least abundant form of the element, at only 5%.

So it looks as if northern nitrate is a legacy of human activity. “People have had an even greater effect than we thought,” says Perakis.

“It’s an amazingly powerful message,” says ecosystems researcher Knute Nadelhoffer of the Marine Biological Laboratory at Woods Hole, Massachusetts. “It changes the way we think about the nitrogen baseline in pristine environments.”

It’s a good reminder of how much pollution has altered northern ecosystems, comments Bridget Emmett, who studies nitrogen pollution at the Centre for Ecology and Hydrology in Bangor, Wales. “Whether the baseline in Chile is the same as the baseline would have been in northern systems is debatable,” she cautions.

Cycle path

Nitrogen makes up about 80% of the air, but only a few bacteria can turn the gas into a form that plants can use.

Over the past century, nitrate fertilizers and nitric oxides emitted from the burning of fossil fuels have roughly doubled the amount of nitrogen available to the biosphere. And farmers are still adding nitrogen to the land, particularly in developing countries.

Plants and microbes have taken up most of the slack. But large quantities of nitrogen are still washed into rivers and the sea, either because the element is not used or because it is released through death or leaf fall.

In many freshwater, estuarine and coastal environments, such as the Gulf of Mexico, this fertilization has changed the range of plants and animals that live there. Nitrogen-loving species swamp others more suited to poorer conditions (the same thing happens in a fertilized lawn). The extra nitrogen can also feed suffocating blooms of algae.

We need to think about nitrogen emissions in the same way that we consider the influence of sulphur on acid rain, or carbon on the climate, says Nadelhoffer.

“We have perturbed the nitrogen cycle much more than the carbon cycle,” he says. “Most forests are still retaining more nitrogen than they release. The question is how much they can retain and for how long.”

The theory that seeks to answer these questions currently hinges on nitrate. The new finding might force a rethink. “It seems that our models might be biased,” Perakis concludes.

References

1. Perakis, S. S. & Hedin, L. O. Nitrogen loss from unpolluted South American forests mainly via dissolved organic compounds. Nature, 415, 416 – 419, (2002).