292
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
1995), plant production (Aber et al. 1993), microbial communities (Arnolds 1989; Munzenberger et
al. 1995), and biogeochemical cycles (Aber et al.
1993; Feijtel et al. 1989; Vitousek et al. 1997; van
Breemen and van Dijk 1988; Friedland et al. 1991).
Nutrient addition experiments are critical tools in
helping ecosystem ecologists determine critical
thresholds of system responses to increased atmospheric deposition. For example, N or S deposition
can acidify the soil, leading to aluminum toxicity
and a loss of essential nutrients such as calcium
(Ca), which has been linked to the decline of forest
systems (Lawrence et al 1995; Fernandez and Rustad 1990; Forsius et al. 1995; Wilson and Skeffington 1994).
The increase in nutrient deposition has made an
understanding of ecosystem nutrient limitation and
its thresholds critical in mitigating human perturbations of the global system. The ability of a system
to retain N plays a crucial role at the regional
and global scales. Nitrogen leaching into groundwater pollutes streams and lakes, while N trace gas
losses can increase tropospheric ozone and contribute to stratospheric ozone depletion and global
warming (Schlesinger 1997). The response of plant
biomass to nutrient additions is critical in determining the capacity of terrestrial systems to serve
as a carbon (C) sink for elevated carbon dioxide
(C02) emissions.
Nutrient addition experiments are becoming increasingly important in addressing applied issues
with important economic and health consequences.
For example, Scandinavian tundra and forest tundra
are typically dominated by lichens, which serve as
the major winter food for reindeer, which in tum
are an important cultural and economic resource for
Saami people. Nutrient addition experiments show
that lichens are extremely sensitive to N deposition.
The results of these experiments suggest that the
pollutants carried northward from eastern Europe
seriously threaten the reindeer industry of Scandinavia (Woodin 1997).
The capacity of ecosystems to retain or "filter"
nutrients is also an important applied issue that can
be addressed by nutrient addition experiments.
Wetlands and riparian zones have been suggested
as natural filters for nutrient-rich waters from agriculture or sewage treatment plants (Jenssen et al.
1994; Rogers et al. 1991; Daniels and Gilliam
1996). Once the nutrient retention capacity of any
ecosystem is exceeded, the nutrients leach into
groundwater. Groundwater with a high nitrate concentration is a health concern, because the nitrate
can be converted to nitrite, which is toxic in lowoxygen environments (such as a human fetus,
where fetal hemoglobin is a strong competitor for
oxygen) (Bouchard et al. 1992; Fan and Steinberg
1996).
Nature of Nutrient Limitation
It is not always clear exactly what is meant by the
phrase nutrient limitation; it has been used to describe the response of various components of the
overall ecosystem to nutrient additions. Historically, the most common test of nutrient limitation
is to determine if the growth of a given plant community is limited by a particular nutrient. In this
case, the initial growth response of plants indicates
whether this plant community is nutrient-limited,
and biomass measurements must be taken before
the plant community changes. However, even
though the overall community biomass may respond to one nutrient, some component species
may be limited by others (Bobbink 1991; Jones and
Martin 1964), and thus nutrient limitation within a
given site can vary among patches with different
species composition.
Altered nutrient supply usually changes competitive balance, leading to changes in species composition. For example, in Alaskan tussock tundra,
most species responded favorably to nutrient addition for the first three years of the experiment, but
after nine years, tall shrubs, which benefited most
from the nutrient addition, produced a dense canopy that reduced the growth of mosses and evergreen shrubs (Chapin et al. 1995). Komer (Komer
1995; Komer and Larcher 1988; Komer 1989; Korner 1999) argues that this competitive exclusion of
low-nutrient-adapted species following addition of
nutrients implies that these species are not nutrientlimited. We argue that the concept of nutrient limitation for individual species or a given community
is most useful as a gauge of the initial growth response of plants to nutrient addition prior to
changes in competitive balance. In contrast, the
changes in the plant community in response to nutrient additions must be incorporated into the concept of nutrient limitation of an ecosystem, since
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
1995), plant production (Aber et al. 1993), microbial communities (Arnolds 1989; Munzenberger et
al. 1995), and biogeochemical cycles (Aber et al.
1993; Feijtel et al. 1989; Vitousek et al. 1997; van
Breemen and van Dijk 1988; Friedland et al. 1991).
Nutrient addition experiments are critical tools in
helping ecosystem ecologists determine critical
thresholds of system responses to increased atmospheric deposition. For example, N or S deposition
can acidify the soil, leading to aluminum toxicity
and a loss of essential nutrients such as calcium
(Ca), which has been linked to the decline of forest
systems (Lawrence et al 1995; Fernandez and Rustad 1990; Forsius et al. 1995; Wilson and Skeffington 1994).
The increase in nutrient deposition has made an
understanding of ecosystem nutrient limitation and
its thresholds critical in mitigating human perturbations of the global system. The ability of a system
to retain N plays a crucial role at the regional
and global scales. Nitrogen leaching into groundwater pollutes streams and lakes, while N trace gas
losses can increase tropospheric ozone and contribute to stratospheric ozone depletion and global
warming (Schlesinger 1997). The response of plant
biomass to nutrient additions is critical in determining the capacity of terrestrial systems to serve
as a carbon (C) sink for elevated carbon dioxide
(C02) emissions.
Nutrient addition experiments are becoming increasingly important in addressing applied issues
with important economic and health consequences.
For example, Scandinavian tundra and forest tundra
are typically dominated by lichens, which serve as
the major winter food for reindeer, which in tum
are an important cultural and economic resource for
Saami people. Nutrient addition experiments show
that lichens are extremely sensitive to N deposition.
The results of these experiments suggest that the
pollutants carried northward from eastern Europe
seriously threaten the reindeer industry of Scandinavia (Woodin 1997).
The capacity of ecosystems to retain or "filter"
nutrients is also an important applied issue that can
be addressed by nutrient addition experiments.
Wetlands and riparian zones have been suggested
as natural filters for nutrient-rich waters from agriculture or sewage treatment plants (Jenssen et al.
1994; Rogers et al. 1991; Daniels and Gilliam
1996). Once the nutrient retention capacity of any
ecosystem is exceeded, the nutrients leach into
groundwater. Groundwater with a high nitrate concentration is a health concern, because the nitrate
can be converted to nitrite, which is toxic in lowoxygen environments (such as a human fetus,
where fetal hemoglobin is a strong competitor for
oxygen) (Bouchard et al. 1992; Fan and Steinberg
1996).
Nature of Nutrient Limitation
It is not always clear exactly what is meant by the
phrase nutrient limitation; it has been used to describe the response of various components of the
overall ecosystem to nutrient additions. Historically, the most common test of nutrient limitation
is to determine if the growth of a given plant community is limited by a particular nutrient. In this
case, the initial growth response of plants indicates
whether this plant community is nutrient-limited,
and biomass measurements must be taken before
the plant community changes. However, even
though the overall community biomass may respond to one nutrient, some component species
may be limited by others (Bobbink 1991; Jones and
Martin 1964), and thus nutrient limitation within a
given site can vary among patches with different
species composition.
Altered nutrient supply usually changes competitive balance, leading to changes in species composition. For example, in Alaskan tussock tundra,
most species responded favorably to nutrient addition for the first three years of the experiment, but
after nine years, tall shrubs, which benefited most
from the nutrient addition, produced a dense canopy that reduced the growth of mosses and evergreen shrubs (Chapin et al. 1995). Komer (Komer
1995; Komer and Larcher 1988; Komer 1989; Korner 1999) argues that this competitive exclusion of
low-nutrient-adapted species following addition of
nutrients implies that these species are not nutrientlimited. We argue that the concept of nutrient limitation for individual species or a given community
is most useful as a gauge of the initial growth response of plants to nutrient addition prior to
changes in competitive balance. In contrast, the
changes in the plant community in response to nutrient additions must be incorporated into the concept of nutrient limitation of an ecosystem, since