In most natural systems nitrogen is a limiting nutrient for plant growth. Though it makes up the majority of our atmosphere it isn't in a plant available form; plants can't just "breathe" it like they do CO2. In the last few decades nitrogen has increased due to the use of fertilizers and the burning of fossil fuels, which emit compounds that end up as plant available nitrogen when brought back to earth with rain.

There are good points and bad points in this.

The researchers analyzed the responses to nitrogen fertilization of 967 plant species. The ecosystems in which they conducted their experiments included arctic and alpine tundra, grasslands, abandoned agricultural fields, and coastal salt marsh communities. While the researchers found that rare plants were vulnerable to nitrogen fertilization, they determined that other plant traits also put even the most abundant plant species at risk: short height (short plants receive less sunlight in the midst of taller plants); the ability to convert atmospheric nitrogen, via bacteria, into a form that plants can use (the cost of supporting the bacteria hurts the plants); and a short life span (longer-living plants do not have to start the life cycle all over again). "Based on simple plant traits, we are able to predict which types of species will be most at risk as nitrogen levels continue to increase," Suding said.

Although it is the most abundant element in the atmosphere, nitrogen from the air can be used by plants only when it is chemically transformed, or "fixed," into compounds that plants can metabolize. In nature, only certain bacteria and algae (and, to a lesser extent, lightning) have the ability to fix atmospheric nitrogen, and the amount they make available to plants is relatively small -- a precious commodity in most terrestrial ecosystems.

"Ecosystems are able to absorb a limited amount of additional nitrogen by producing more plant mass, just as garden vegetables do when fertilized," Suding said. "Some species may be better able to take advantage of this added resource, getting bigger at the expense of other species and causing diversity to decline." . . .

The researchers added nitrogen fertilizers experimentally at sites in all the ecosystems they studied. Suding explained that even without the fertilizers, nitrogen availability is on the increase at all the sites due to atmospheric deposition -- a process by which gases or particles are transferred from the atmosphere to the Earth's surface. "Nitrous oxides from fossil fuel consumption fall back to Earth as dry particles and in rain," she said. "Annual nitrogen deposition rates can reach more than 50 kilograms per hectare in auto-dominated areas like Southern California, which is in the range of application rates of nitrogen fertilizers for farming. Even relatively pristine areas such as the alpine tundra are experiencing substantial inputs of nitrogen falling from the sky.

"Our results predict that the impacts of nitrogen fertilization are widespread and dramatic, and that many species face local extinction risk. This work will help us identify species most at risk and point to management strategies to protect our ecosystems in face of these impacts."

In many natural systems water is a variable as well as limiting nutrient. More nitrogen has limited effects since without water it isn't helpful. Plants have evolved to use variable nutrients as part of their competitive survival strategies. Growing taller, faster, or getting a head start early in the season by doing better in cool temperatures are examples of such strategies. They sometimes steal a march on other species and over top them, stealing their light.

This is an issue for pasture managers since tall plants are less nutritious and digestible as they have a higher lignin content to stiffen the stalks. Even worse, a tall plant switches from vegetative production to flower and seed production, stealing energy from leaves to use for reproduction. Gone to seed means loss of nutrition and palatability for leaves and stems.

So it's a race during flush times to keep swards grazed down so that plants remain vegetative, extending their lives by clipping them down, forcing them to regrow from root starch reserves and keep photosynthesizing rather than going to seed and becoming dead or dormant. Grazers naturally assist this process when free to roam since they value lush new growth and will eat more of it. It also allows shorter species to have a fair shot at survival, increasing the diversity of the sward and so its total productivity and nutritional value. The sward thickens, with more but shorter individuals rather than fewer but taller individuals. This makes better use of the biotope space below ground.

I'd find this new research more useful if it dealt with the effects of grazing as well as nitrogen. In most natural settings this would be a major factor since everything from rabbits to elk, even migrating geese and meadow voles, graze lush growth preferentially. Some of them, such as the voles, are capable of rapid population increase in response to increased forage availability. Concluding that "many species face local extinction risk" as a consequence of increased nitrogen availability isn't convincing without considering a complete ecosystem including predators.