19. Nutrient Manipulations in Terrestrial Ecosystems
Fixation of NUt on minerals is not a pennanent
loss-it is often a temporary retention mechanism
that decreases nitrification and leaching loss. Ammonium fixation on soil is most common on coarse
clays and fine silt, especially on vermiculite and
illite. Fixation increases with freezing and drying.
Soils with high concentrations of K and Ca tend to
have very low NHt adsorption.
Common Nitrogen Fertilizers
Nitrogen is commonly added as NHt salts, NO;
salts, or urea. Ammonium nitrate (NH4N03) is a
very common fonn of N fertilizer in nutrient addition experiments. Additions of this fonn ensure
high N availability, without the confounding factor
of adding other mineral nutrients, and it has little
effect on soil pH. Disadvantages to using NH4N0 3
are that it is more prone to denitrification and leaching than NHt salts, it must be stored carefully because it can absorb moisture and cake very easily,
and, if it comes into contact with oxidizable C
sources (such as fuel), it is very explosive, so it
must be handled with care.
Ammonium salts have the best system retention
of N in most cases, but they reduce pH and should
not be used in acidic soils, except in combination
with urea, which has the counteracting effect of
raising pH. Care must also be taken in applying
NHt salts to calcareous soils (Kissel et al. 1985);
NHt salts that precipitate with Ca (fluoride [F-],
sulfate [SOi], phosphate [POi]) can stimulate
NH3 volatilization and nitrification, leading to high
loss rates of the N added. However, NHt additions
with chloride (Cl-), NO;, and iodide (1-) are suitable for application to calcareous soils (Tisdale et
al. 1993). In addition, at high pH, additions of
NHt can lead to the accumulation of nitrite
(NO;), which is toxic to plants and microbes. Ammonium sulfate is the most acidic N addition (Boswell 1985), and should be avoided in low pH soils.
Ammonium phosphate can be used as a fertilizer in
situations in which both Nand P additions are required. Ammonium chloride (NH4Cl) can be problematic because some plants are sensitive to it at
relatively low concentrations, and at high concentrations, chloride can be toxic to all plants. This is
especially an issue in long-tenn additions, or areas
where Cl- can accumulate quickly (such as arid
regions with high evaporation).
297
Nitrate salts are not as acid as ammonium salts.
Nitrate accompanied by base cations (e.g., sodium
[Na+], Ca 2 +) can increase pH, and should be
avoided in basic soils. Nitrate fertilizers are very
soluble and mobile and are subject to high loss rates
through leaching and denitrification. However, in
some systems, long-tenn fertilization studies have
seen higher plant yields in response to NO; than
NHt due to the negative effect of NHt on soil
acidity (Koren'kov 1983). Common sources of nitrate are NaN03, KN03, and Ca(N03h.
Urea is one of the most common fonns of N
additions. It is relatively inexpensive and easy to
handle, it has much less tendency to absorb moisture and cake than ammonium nitrate, and it is not
prone to explosion. However, there are many problems associated with urea additions, and careful attention must be paid to the characteristics of a system before deciding that its use is appropriate. Urea
should never be added to soils with high pH (pH
greater than 7.5). As with ammonium applications,
high pH can lead to nitrite accumulation, as well as
high rates of ammonia volatilization. This can occur with urea applications even in acid or neutral
soils, because urea can substantially increase soil
pH (Bremner 1995).
Another problem associated with urea application is that it often contains biuret, an impurity that
inhibits germination and seedling growth. Because
of this, as well as the toxicity of high concentrations
of NH3 to plants, urea should not be added just
before germination, or early in the growing season.
Urea losses can be minimized by subsurface applications, rather than a broadcast surface application.
Another important option for N additions is
slow-release fertilizers, which are the easiest way
to apply N gradually and continuously. Slowreleasing N is available in a number of different
fonns, but it is most commonly supplied as urea,
particularly in the slow-releasing fertilizers that do
not also supply P and K (Hauck 1985).
How to Add Nitrogen Fertilizers
As discussed above, the fonn of N chosen will dictate what conditions minimize losses and negative
effects of fertilizer N. In general, N fertilization is
optimized when added at the time of peak plant
uptake. Nitrate additions should be avoided in areas
with high rainfall, or that are very wet, in order to
Fixation of NUt on minerals is not a pennanent
loss-it is often a temporary retention mechanism
that decreases nitrification and leaching loss. Ammonium fixation on soil is most common on coarse
clays and fine silt, especially on vermiculite and
illite. Fixation increases with freezing and drying.
Soils with high concentrations of K and Ca tend to
have very low NHt adsorption.
Common Nitrogen Fertilizers
Nitrogen is commonly added as NHt salts, NO;
salts, or urea. Ammonium nitrate (NH4N03) is a
very common fonn of N fertilizer in nutrient addition experiments. Additions of this fonn ensure
high N availability, without the confounding factor
of adding other mineral nutrients, and it has little
effect on soil pH. Disadvantages to using NH4N0 3
are that it is more prone to denitrification and leaching than NHt salts, it must be stored carefully because it can absorb moisture and cake very easily,
and, if it comes into contact with oxidizable C
sources (such as fuel), it is very explosive, so it
must be handled with care.
Ammonium salts have the best system retention
of N in most cases, but they reduce pH and should
not be used in acidic soils, except in combination
with urea, which has the counteracting effect of
raising pH. Care must also be taken in applying
NHt salts to calcareous soils (Kissel et al. 1985);
NHt salts that precipitate with Ca (fluoride [F-],
sulfate [SOi], phosphate [POi]) can stimulate
NH3 volatilization and nitrification, leading to high
loss rates of the N added. However, NHt additions
with chloride (Cl-), NO;, and iodide (1-) are suitable for application to calcareous soils (Tisdale et
al. 1993). In addition, at high pH, additions of
NHt can lead to the accumulation of nitrite
(NO;), which is toxic to plants and microbes. Ammonium sulfate is the most acidic N addition (Boswell 1985), and should be avoided in low pH soils.
Ammonium phosphate can be used as a fertilizer in
situations in which both Nand P additions are required. Ammonium chloride (NH4Cl) can be problematic because some plants are sensitive to it at
relatively low concentrations, and at high concentrations, chloride can be toxic to all plants. This is
especially an issue in long-tenn additions, or areas
where Cl- can accumulate quickly (such as arid
regions with high evaporation).
297
Nitrate salts are not as acid as ammonium salts.
Nitrate accompanied by base cations (e.g., sodium
[Na+], Ca 2 +) can increase pH, and should be
avoided in basic soils. Nitrate fertilizers are very
soluble and mobile and are subject to high loss rates
through leaching and denitrification. However, in
some systems, long-tenn fertilization studies have
seen higher plant yields in response to NO; than
NHt due to the negative effect of NHt on soil
acidity (Koren'kov 1983). Common sources of nitrate are NaN03, KN03, and Ca(N03h.
Urea is one of the most common fonns of N
additions. It is relatively inexpensive and easy to
handle, it has much less tendency to absorb moisture and cake than ammonium nitrate, and it is not
prone to explosion. However, there are many problems associated with urea additions, and careful attention must be paid to the characteristics of a system before deciding that its use is appropriate. Urea
should never be added to soils with high pH (pH
greater than 7.5). As with ammonium applications,
high pH can lead to nitrite accumulation, as well as
high rates of ammonia volatilization. This can occur with urea applications even in acid or neutral
soils, because urea can substantially increase soil
pH (Bremner 1995).
Another problem associated with urea application is that it often contains biuret, an impurity that
inhibits germination and seedling growth. Because
of this, as well as the toxicity of high concentrations
of NH3 to plants, urea should not be added just
before germination, or early in the growing season.
Urea losses can be minimized by subsurface applications, rather than a broadcast surface application.
Another important option for N additions is
slow-release fertilizers, which are the easiest way
to apply N gradually and continuously. Slowreleasing N is available in a number of different
fonns, but it is most commonly supplied as urea,
particularly in the slow-releasing fertilizers that do
not also supply P and K (Hauck 1985).
How to Add Nitrogen Fertilizers
As discussed above, the fonn of N chosen will dictate what conditions minimize losses and negative
effects of fertilizer N. In general, N fertilization is
optimized when added at the time of peak plant
uptake. Nitrate additions should be avoided in areas
with high rainfall, or that are very wet, in order to