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 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.
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