Nitrogen in unpolluted streams is more organic.
Agriculture has added to the natural nitrogen cycle.
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.
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.