BioEd Online

Nitrogen emitted into the atmosphere as a result of human activity could be changing plant communities worldwide. Although hot spots of high nitrogen pollution are known to change plant dynamics, new work predicts substantial effects from the lower, chronic levels of pollution found throughout most of the world.

A study of a prairie site in the middle of Minnesota shows that after more than 20 years of slow, chronic deposition of nitrogen — at levels typical of nitrogen pollution in most of the industrial world — cut the number of plant species by 17% compared with control plots not exposed to extra nitrogen.

The losers under this nitrogen regimen tended to be tender herb-like species, the researchers report in Nature¹. David Tilman, of the University of Minnesota in Saint Paul and one of the authors of the work, says that his personal favourites among the victims are the showy grey goldenrod and the light-purple blazing star.

The good news is that some of the diversity can be recovered if nitrogen pollution is reduced, they say. Plots that stopped receiving additional nitrogen in 1991 slowly recovered and began to look much like the control plots, as seeds from the lost species migrated back in from other areas. But Tilman cautions that without such reservoirs of plant diversity handy, recovery could take much longer than the 13 years it took in this study.

Spread it on thick

Humans spread a lot of nitrogen around. The element is a key limiting factor in the growth of many plants, so nitrogen-based fertilizers have been developed to enable people to grow much more food. It is also produced by the burning of fossil fuels and from the excreta of livestock and it has a way of wandering into the atmosphere and raining down on areas that are superficially untouched by humans.

Nitrogen falling on the land has risen from a pre-industrial levels of 1-3 kilograms per hectare per year to 7-100 kilograms per hectare per year, depending on the local intensity of nitrogen use. Scientists have estimated that human intervention today more than doubles the amount of nitrogen moving from the atmosphere to Earth each year².

Heavy doses of nitrogen are known to have a big effect on landscapes: it frees up plants previously limited by the scarcity of the element to grow more vigorously, often at the expense of others. As nitrogen-thrifty plants are overwhelmed by the "faster-growing weedier things" in the area, says Tilman, the number of species in an area drops.

Now, Tilman and his co-author Christopher Clark have shown that even very low levels of nitrogen can affect diversity. Their study looked at levels of nitrogen pollution as low as 10 kilograms per hectare per year, on top of the the ambient background level of 6 kilograms per hectare per year in this area. That's a level of pollution typical of much of the industrial world. "We saw that these very low rates of nitrogen deposition were having a cumulative effect very similar to the shorter-term effects seen at higher levels," says Tilman.

The team found no threshold at which added nitrogen did not affect plant diversity. "Low rates of nitrogen deposition matter much more than everyone thought," says Tilman.

The wrong end of a cow

Such effects are expected to be widespread. As the nitrogen released by "factories and automobiles in one area of the country is carried by the wind to far-flung places, virtually every place in the world — including our national parks and other natural landmarks — now receive elevated nitrogen due to air pollution," says Katharine Suding, an ecologist at the University of California, Irvine. The new work, she says, "indicates that we can expect losses to grow worse and worse over time as nitrogen accumulates unless we enact policies to curb the rise of nitrogen deposition."

Roland Bobbink, a landscape ecologist at the University of Utrecht, says he has seen this happening in the Netherlands, where the use of fertilizers in its heyday in the 1980s famously boosted nitrogen levels to 100 kilograms of nitrogen per year per hectare. Plant communities changed dramatically, he says: the heath turned to grassland, and is now starting to turn to forest. Since then, a suite of environmental policies have lowered the nitrogen deposition there — but not to zero.

Tilman says that what was seen in the Netherlands is happening worldwide, albeit more slowly. This makes it clear that humankind's love affair with the seventh element needs tempering, he says. Over-fertilization must be ended, and concentrated cattle operations, with their nitrogen-emitting manure lagoons, might have to be reconsidered.

"To me this is one of the big ways that humans are changing the world, but it is not very well understood by policymakers," says Tilman.