Nitrogen fertilization of soil puts rare plant species at risk, nationwide study determines

Rare plant species are six times more likely than abundant species to be lost due to nitrogen fertilization of soil, UC Irvine biologist Katharine Suding and colleagues have found through experiments conducted across nine ecosystems in North America. While nitrogen increases the production of plants, an excess amount of it, the researchers conclude, creates a competition among plants for space that drives rare plants – plants that are uncommon and not abundant – out of existence, causing a loss of biodiversity in the ecosystems.

The researchers reported their findings in the March 22 issue of the Proceedings of the National Academy of Sciences.

“The results from the 34 nitrogen-fertilization experiments are useful for putting together conservation strategies that protect rare plants and spare them from extinction,” said Suding, an assistant professor of ecology and evolutionary biology, and the first author of the paper. “As a basic building block of plant and animal proteins, nitrogen is a nutrient essential to all forms of life. But it is possible to have too much of a good thing. Driven by an increase in the use of fertilizers and the burning of fossil fuels, the amount of nitrogen available to plants at any given time has more than doubled since the 1940s. This high level of nitrogen addition appears to be having a very large negative impact on diversity, jeopardizing the existence of some types of species.”

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.”

Examples of biodiversity loss due to nitrogen fertilization:

• In the sand prairie in the northern Midwest, species richness declined 50 percent and bunch grasses were replaced by invasive, weedy European grasses. Many of the species lost are native species with a short stature. They get “shaded out” by the aggressive exotic species.
• In the tallgrass prairie in Kansas, an exotic grass takes over due to fertilization. Over half of the legumes (species that form a symbiotic relationship with bacteria to fix nitrogen from the atmosphere and so do not rely on soil nitrogen) are lost because the benefits associated with nitrogen-fixing no longer outweigh the costs. These species include plants in the pea family such as clovers.
• In California, fertilization gives a further advantage to the exotic annual grasses that already cover much of the hillsides. The wildflower species (similar to California poppies or goldfields) are lost in the annual grasslands.
• In the arctic tundra of Alaska, a birch shrub increases five-fold due to nitrogen fertilization, and diversity plummets to a handful of species.

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.”

Suding’s co-authors of the PNAS paper are Scott L. Collins, University of New Mexico, Albuquerque; Laura Gough, University of Texas at Arlington; Christopher Clark, University of Minnesota, St. Paul; Elsa E. Cleland, Stanford University; Katherine Gross, Michigan State University, Hickory Corners; Daniel G. Milchunas, Colorado State University, Fort Collins; and Steven Pennings, University of Houston.

Currently, the researchers are working on what controls the sensitivity of the different ecosystems to nitrogen fertilization. “Some systems appear to buffer the increase in nitrogen – with less of a diversity crash than others – and we want to know why,” Suding said.

The research was supported by the National Science Foundation.

About the University of California, Irvine: The University of California, Irvine is a top-ranked public university dedicated to research, scholarship and community service. Founded in 1965, UCI is among the fastest-growing University of California campuses, with more than 24,000 undergraduate and graduate students and about 1,400 faculty members. The second-largest employer in dynamic Orange County, UCI contributes an annual economic impact of $3 billion.