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

[Patricia_Ford at fws.gov]

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
www.uci.edu


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