Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

Nitrogen in the Soil

As with water and air, reactive nitrogen builds up in soil. There's a limit, however, to how much nitrogen plants can use. When soil reaches a point at which plants can't use additional nitrogen, it's said to be "saturated." And saturated soil, in theory at least, will shed any additional nitrogen introduced to it. But that nitrogen doesn't leave unaccompanied. "When it leaches out of the system," says Townsend, "it takes other nutrients with it, so it ends up acidifying the soil, and it takes things like magnesium and calcium out into the water. And you end up with a very unbalanced system."

If it's true that saturated soil immediately passes additional nitrogen, rather than denitrifying it, that could be bad news for the near future, says Howarth, with all that excess nitrogen flowing straight to groundwater, rivers, streams, and seas. However, he says, we have a very poor understanding of what is actually happening. "If the nitrogen is accumulating in soil, it could be a temporary phenomenon until it saturates the ability to store it. Then we have a much bigger problem," he says. "If it is being denitrified, on the other hand, that's more of a steady-state process, and it can probably continue to do that."

Townsend says some scientists had hoped that excess reactive nitrogen levels might actually reduce greenhouse gases by stimulating plant growth, which locks up carbon dioxide. But, he says, "It doesn't seem likely that it's going to play a dominant role." Although the jury is still out, Tilman adds, "there isn't very good evidence that nitrogen deposition actually does lead to increased carbon removal and storage."

Although more reactive nitrogen means more growth, it also changes which of the species in an ecosystem thrive. For example, in grasslands that received increased nitrogen, Tilman says, "the species composition changed to plants that had litter that decomposed more quickly. And because it decomposed more quickly, there was actually no net storage of carbon with added nitrogen."

On the surface it could seem as though additional nutrition might at least help struggling ecosystems thrive. In fact, however, reactive nitrogen can disrupt an ecosystem's delicate balance. "From the 1850s on, we've known that the addition of nutrients to terrestrial ecosystems causes changes in which species are there and causes a loss of diversity in those systems," says Tilman. "Under the highest rates of agriculturally driven nitrogen that we've seen, there's a very strong effect [on biodiversity loss]." Recent field studies in Great Britain--reported by Open University Earth scientist Carly J. Stevens and colleagues in the 19 March 2004 issue of Science--have confirmed that biodiversity decreased as unaided nitrogen deposition increased in a sample of 68 grasslands. Tilman's experimental work in which nitrogen was added to ecosystems shows similar results, he says.

Land Use and Nitrogen in Runoff