296
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
Nutrients are typically added in solid form, although this can bum the surface vegetation. For
example, mosses, lichens, and many evergreen
shrubs are sensitive to high nutrient addition rates.
However, addition of dissolved nutrients in a water
solution is time consuming and can create artifacts
if moisture is potentially limiting to growth.
Use of slow-release fertilizer (e.g., Osmocote or
rock phosphate) is an excellent way to provide nutrients gradually, rather than in a single pulse. These
fertilizers differ in the forms of nutrients present
(ammonium vs. nitrate), the combinations ofnutrients supplied, and in the rate of release. In other
cases, pulsed nutrient input (as simulated by addition of soluble nutrients) may be more typical of
natural ecosystem patterns (Cui and Caldwell 1997;
Bilbrough and Caldwell 1997).
Ideally, the nutrients should be added to that
component of the ecosystem where the processes
naturally occur. For example, if the purpose is to
trace N uptake by plants and soil microorganisms,
isotopically labeled N should be distributed uniformly through the rooting zone rather than applied
to the surface, where it might contaminate the
plants or be immobilized by surficial vegetation (algae or mosses) or litter microbes. These experiments are most readily done with microcosms, in
which the isotope can be injected into the soil using
long "spinal-tap" needles that are soldered closed
at the end but have holes drilled in the side of the
needle (Jackson et al. 1989; Hungate et al. 1997).
Nadelhoffer et al. (1995) have applied 15N to the
litter layer of forests to follow the fate of nutrients
returned to the forest floor in litterfall.
Form of Nutrients Added
Nutrients should be added in a chemical form that
reasonably approximates the form that is available
in that ecosystem. For example, N might be added
as nitrate in ecosystems with high nitrification rates
or as urea or NHi in ecosystems with low nitrification rates. In N deposition experiments, it is important to choose the form of N that is being deposited at the study sites (e.g., NO; in the eastern
United States, NHt in the Netherlands), because
these different forms of N can have very different
effects on ecosystem and community response
(Crabtree and Bazzaz 1993; Vaast and Zasoski
1992).
The form of added nutrients is important, because inappropriate choices can create unintended
side effects. For example, addition of ammonium
chloride or sodium nitrate can cause chloride or sodium toxicity, create osmotic problems at high levels, or alter soil pH. Two options are to add ammonium nitrate or to add two ammonium forms
which differ in their pH effects (see below). Which
form is chosen also determines the fate and retention of fertilizer, and effects on ecosystem and community dynamics.
Many fertilizers contain multiple elements. For
example, most commercial phosphate fertilizers are
made with sulfuric acid and contain enough sulfate
to act as a S fertilizer in S-deficient soils. When
choosing which form of fertilizer to use, pay careful
attention to the other nutrients that are in the fertilizer; nutrients are commonly complexed to forms
of other limiting nutrients (K, N, P, S, Ca), which
could also play an important role in biomass
response.
Practicality and economy are two other important considerations when choosing the appropriate
form of fertilizer. For example, studies requiring
large P inputs would necessitate as much as a 30cm-deep layer of rock phosphate to achieve the desired P additions! Nutrient addition experiments
that focus on agriculture or rangeland applications
need to consider economy as well. Osmocote, or
any other expensive nutrient source, would never
be used by growers.
Nitrogen
Control of Fertilizer Nitrogen Availability
The major processes that decrease the fertilizer N
available to plants are microbial immobilization,
leaching loss, trace gas losses through nitrification
and denitrification, ammonia volatilization, and adsorption ofNHt on soil particles. To minimize loss
through leaching and denitrification, it is best to
avoid additions of N during very wet times. Ammonia (NH 3 ) volatilization can lead to substantial
losses of added N, as high as 25 to 50% (Tisdale et
al. 1993). Ammonia losses are especially high in
systems with high soil pH, soil moisture around
field capacity, high evaporation, and substantial
plant residue at the surface of the soil. In such systems, N should be added as NO; rather than as
NHt or urea.
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
Nutrients are typically added in solid form, although this can bum the surface vegetation. For
example, mosses, lichens, and many evergreen
shrubs are sensitive to high nutrient addition rates.
However, addition of dissolved nutrients in a water
solution is time consuming and can create artifacts
if moisture is potentially limiting to growth.
Use of slow-release fertilizer (e.g., Osmocote or
rock phosphate) is an excellent way to provide nutrients gradually, rather than in a single pulse. These
fertilizers differ in the forms of nutrients present
(ammonium vs. nitrate), the combinations ofnutrients supplied, and in the rate of release. In other
cases, pulsed nutrient input (as simulated by addition of soluble nutrients) may be more typical of
natural ecosystem patterns (Cui and Caldwell 1997;
Bilbrough and Caldwell 1997).
Ideally, the nutrients should be added to that
component of the ecosystem where the processes
naturally occur. For example, if the purpose is to
trace N uptake by plants and soil microorganisms,
isotopically labeled N should be distributed uniformly through the rooting zone rather than applied
to the surface, where it might contaminate the
plants or be immobilized by surficial vegetation (algae or mosses) or litter microbes. These experiments are most readily done with microcosms, in
which the isotope can be injected into the soil using
long "spinal-tap" needles that are soldered closed
at the end but have holes drilled in the side of the
needle (Jackson et al. 1989; Hungate et al. 1997).
Nadelhoffer et al. (1995) have applied 15N to the
litter layer of forests to follow the fate of nutrients
returned to the forest floor in litterfall.
Form of Nutrients Added
Nutrients should be added in a chemical form that
reasonably approximates the form that is available
in that ecosystem. For example, N might be added
as nitrate in ecosystems with high nitrification rates
or as urea or NHi in ecosystems with low nitrification rates. In N deposition experiments, it is important to choose the form of N that is being deposited at the study sites (e.g., NO; in the eastern
United States, NHt in the Netherlands), because
these different forms of N can have very different
effects on ecosystem and community response
(Crabtree and Bazzaz 1993; Vaast and Zasoski
1992).
The form of added nutrients is important, because inappropriate choices can create unintended
side effects. For example, addition of ammonium
chloride or sodium nitrate can cause chloride or sodium toxicity, create osmotic problems at high levels, or alter soil pH. Two options are to add ammonium nitrate or to add two ammonium forms
which differ in their pH effects (see below). Which
form is chosen also determines the fate and retention of fertilizer, and effects on ecosystem and community dynamics.
Many fertilizers contain multiple elements. For
example, most commercial phosphate fertilizers are
made with sulfuric acid and contain enough sulfate
to act as a S fertilizer in S-deficient soils. When
choosing which form of fertilizer to use, pay careful
attention to the other nutrients that are in the fertilizer; nutrients are commonly complexed to forms
of other limiting nutrients (K, N, P, S, Ca), which
could also play an important role in biomass
response.
Practicality and economy are two other important considerations when choosing the appropriate
form of fertilizer. For example, studies requiring
large P inputs would necessitate as much as a 30cm-deep layer of rock phosphate to achieve the desired P additions! Nutrient addition experiments
that focus on agriculture or rangeland applications
need to consider economy as well. Osmocote, or
any other expensive nutrient source, would never
be used by growers.
Nitrogen
Control of Fertilizer Nitrogen Availability
The major processes that decrease the fertilizer N
available to plants are microbial immobilization,
leaching loss, trace gas losses through nitrification
and denitrification, ammonia volatilization, and adsorption ofNHt on soil particles. To minimize loss
through leaching and denitrification, it is best to
avoid additions of N during very wet times. Ammonia (NH 3 ) volatilization can lead to substantial
losses of added N, as high as 25 to 50% (Tisdale et
al. 1993). Ammonia losses are especially high in
systems with high soil pH, soil moisture around
field capacity, high evaporation, and substantial
plant residue at the surface of the soil. In such systems, N should be added as NO; rather than as
NHt or urea.