19
Nutrient Manipulations
in Terrestrial Ecosystems
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
Introduction
Nutrient addition experiments are the ecosystem
manipulations that are undertaken most frequently,
because they are relatively easy and inexpensive to
perform. In addition, because of the widespread nature of nutrient limitation in terrestrial ecosystems
(Vitousek and Howarth 1991), these experiments
almost always show dramatic results. As with much
of ecology, the roots of nutrient manipulation experiments come from agriculture, where fertilizer
trials are regularly used to determine which nutrients most strongly limit plant growth and what the
biological and economic returns are for different
levels of nutrient addition.
Ecological Questions Addressed
by Nutrient Addition
Most early nutrient addition experiments in ecology
addressed questions that were similar to those of
agronomists: Are nutrients limiting to plant growth,
or which nutrients most strongly limit plant growth
in a particular location? These experiments demonstrate that nutrients are among the factors that
limit plant growth in most terrestrial ecosystems
(Chapin 1980; Vitousek and Howarth 1991; Shaver
et al. 1986), and that the particular nutrients that are
limiting change through space and time. The extension of these site-specific patterns of nutrient limitation across the landscape have led to an understanding of large-scale ecological issues such as
changes in nutrient availability and limitation
through succession and soil development (Chapin
et al. 1994; Vitousek and Farrington 1997; Walker
and Syers 1976). Nutrient addition experiments
have also been used to elucidate the roles of nutrients in determining plant community dynamics
(Wedin and Tilman 1993; Berendse et al. 1992;
Willems et al. 1993; Bobbink 1991), microbial
characteristics (Lovell and Hatch 1998; Clarholm
1993; Stamford et al. 1997), and ecosystem processes (Bremer and Kurkmar 1997, Magill et al.
1997).
By using isotopically labeled fertilizer, it is possible to track the quantity of added nutrients that is
readily available for plant uptake, versus the
amount that is adsorbed, leached, lost as gas, immobilized by microbes, or incorporated into soil
organic matter. Isotopes also enable us to follow
the dynamics of the added nutrients through time,
tracking how long they are retained and the mechanisms of retention. By comparing plant uptake of
labeled (fertilizer-derived) and unlabeled (native
soil-derived) nutrients, we can gain important insights into how nutrient additions alter nutrient cycling and the role of native soil nutrients in plant
nutrition. Also, by tracing the relative uptake of labeled nutrients by different plant species, we can
determine how plants compete for resources at different nutrient levels.
The role of nutrients as a control over ecosystem
processes has recently received more attention due
to concern over the effects of atmospheric deposition of nitrogen (N), and to a lesser extent, sulfur,
(S), on ecosystems. Nutrient deposition can have
profound effects on plant community composition
(Aerts and Berendse 1998; Gunn 1995; Hogg et al.
291
Nutrient Manipulations
in Terrestrial Ecosystems
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
Introduction
Nutrient addition experiments are the ecosystem
manipulations that are undertaken most frequently,
because they are relatively easy and inexpensive to
perform. In addition, because of the widespread nature of nutrient limitation in terrestrial ecosystems
(Vitousek and Howarth 1991), these experiments
almost always show dramatic results. As with much
of ecology, the roots of nutrient manipulation experiments come from agriculture, where fertilizer
trials are regularly used to determine which nutrients most strongly limit plant growth and what the
biological and economic returns are for different
levels of nutrient addition.
Ecological Questions Addressed
by Nutrient Addition
Most early nutrient addition experiments in ecology
addressed questions that were similar to those of
agronomists: Are nutrients limiting to plant growth,
or which nutrients most strongly limit plant growth
in a particular location? These experiments demonstrate that nutrients are among the factors that
limit plant growth in most terrestrial ecosystems
(Chapin 1980; Vitousek and Howarth 1991; Shaver
et al. 1986), and that the particular nutrients that are
limiting change through space and time. The extension of these site-specific patterns of nutrient limitation across the landscape have led to an understanding of large-scale ecological issues such as
changes in nutrient availability and limitation
through succession and soil development (Chapin
et al. 1994; Vitousek and Farrington 1997; Walker
and Syers 1976). Nutrient addition experiments
have also been used to elucidate the roles of nutrients in determining plant community dynamics
(Wedin and Tilman 1993; Berendse et al. 1992;
Willems et al. 1993; Bobbink 1991), microbial
characteristics (Lovell and Hatch 1998; Clarholm
1993; Stamford et al. 1997), and ecosystem processes (Bremer and Kurkmar 1997, Magill et al.
1997).
By using isotopically labeled fertilizer, it is possible to track the quantity of added nutrients that is
readily available for plant uptake, versus the
amount that is adsorbed, leached, lost as gas, immobilized by microbes, or incorporated into soil
organic matter. Isotopes also enable us to follow
the dynamics of the added nutrients through time,
tracking how long they are retained and the mechanisms of retention. By comparing plant uptake of
labeled (fertilizer-derived) and unlabeled (native
soil-derived) nutrients, we can gain important insights into how nutrient additions alter nutrient cycling and the role of native soil nutrients in plant
nutrition. Also, by tracing the relative uptake of labeled nutrients by different plant species, we can
determine how plants compete for resources at different nutrient levels.
The role of nutrients as a control over ecosystem
processes has recently received more attention due
to concern over the effects of atmospheric deposition of nitrogen (N), and to a lesser extent, sulfur,
(S), on ecosystems. Nutrient deposition can have
profound effects on plant community composition
(Aerts and Berendse 1998; Gunn 1995; Hogg et al.
291