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

Sonya msredsonya at earthlink.net
Mon May 9 01:02:24 CDT 2005

University of California - Irvine 19.04.2005

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.

More information: www.today.uci.edu

Sonya    mailto:msredsonya at earthlink.net
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