300
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
can be high in soils with high clay content and high
concentrations of aluminum oxides and iron oxides.
Adsorption increases at low pH, and is greater in
soils with kaolinite than with illite or montmorillonite. This adsorption is readily reversible and will
only affect short-term S availability, and probably
increase long-term availability by preventing S
leaching. Sulfate can also be removed from the
plant-available pool by precipitating with calcium
carbonate (CaC0 3 ).
Common Sulfur Fertilizers
Like phosphorus, S has two main forms of fertilizer, a readily available water-soluble form and a
slowly available insoluble form.
Sulfate additions are immediately plant available, and supply rates are comparable with
NH 4 N0 3 , KCl, and water-soluble P additions. Different forms of soluble S are approximately equal
in their efficiency to supply S to plants, unless complexed with zinc (Zn), copper (Cu), or Mn. All
forms of water-soluble S are complexed with other
important nutrients (N, P, iron [Fe], Ca, K, Mg), so
careful consideration of other limiting nutrients in
a system must be made before deciding on which
form of S fertilizer to use. Gypsum (CaS04) is one
of the most common ways to add sulfur. It is also
possible to add S as single superphosphate if a combined application with P is desired. In order to minimize S leaching loss, applications of sulphate
should be made close to the time of peak plant
uptake.
The second main form of S fertilizer is elemental
sulfur (So), which is a water-insoluble solid. When
finely ground and mixed with soils, it is oxidized
to SO~- by microbes. The relative effectiveness of
So application is determined by the particle size
added, how and when it is added, the So oxidizing
characteristics of soil, and environmental conditions. Finer-size particles supply SO~- at a faster
rate. A mixture of So particle sizes is sometimes
desirable in order to achieve both rapid and longlasting S availability. Dissolution of SO~- is maximized if the pellets undergo wet-dry or freezethaw cycles. A way to maximize release of
plant-available S is to use irregularly shaped, porous granules. Alternatively, So can be applied with
bentonite, a clay that absorbs water and causes the
So granules to disintegrate.
The timing of additions is crucial, So must be
added as far ahead of germination and growth initiation as possible (Beaton et al. 1985). Even when
adding So with bentonite or porous granules, a few
months are needed for substantial release of plantavailable S. Elemental sulfur applications are more
efficient if they are incorporated into the soil. A
possible problem associated with So additions is
acidification of the soil, but this can be avoided by
making sure that the fertilizer is uniformly distributed (Beaton et al. 1985).
Isotopes
The use of isotopically labeled fertilizer allows
ecologists to ask more refined, mechanistic questions. Isotopes can serve as an invaluable tool to
check the efficiency of nutrient additions by tracking the fate of fertilizer additions, and thus helping
to determine if the lack of response of a system to
nutrient additions is mediated by low fertilizer
availability due to adsorption, loss, or microbial immobilization (Drury and Beauchamp 1991). Isotopes can also be used to look at the long-term fate
and availability of fertilizer (Preston and Mead
1994). Reciprocal transplants of litter from plots
fertilized with labeled and unlabeled nutrients can
help track recycling of fertilizer nutrients from the
litter (Power et al. 1986).
Isotopes can also elucidate the mechanisms of
ecosystem response to nutrient additions. Community composition changes may be explained by
differences in resource competition at different nutrient levels, which can be determined by comparing labeled fertilizer uptake by different species
(Nannipieri et al. 1985; Chang et al. 1996). Comparing amounts of labeled and unlabeled nutrients
in soil and plant pools can elucidate how nutrient
additions alter native soil nutrient availability (Pilbeam et al. 1997; Hart et al. 1986; Clinton and
Mead 1994).
Isotopes can be particularly useful for studying
system response to nutrient deposition. Isotopically
labeled nutrients can elucidate the pathways and
controls over nutrient movement through ecosystems, and can point to the mechanisms of ecosystem retention and loss (Nadelhoffer et al. 1995;
Buchmann et al. 1996; Koopmans et al. 1996).
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
can be high in soils with high clay content and high
concentrations of aluminum oxides and iron oxides.
Adsorption increases at low pH, and is greater in
soils with kaolinite than with illite or montmorillonite. This adsorption is readily reversible and will
only affect short-term S availability, and probably
increase long-term availability by preventing S
leaching. Sulfate can also be removed from the
plant-available pool by precipitating with calcium
carbonate (CaC0 3 ).
Common Sulfur Fertilizers
Like phosphorus, S has two main forms of fertilizer, a readily available water-soluble form and a
slowly available insoluble form.
Sulfate additions are immediately plant available, and supply rates are comparable with
NH 4 N0 3 , KCl, and water-soluble P additions. Different forms of soluble S are approximately equal
in their efficiency to supply S to plants, unless complexed with zinc (Zn), copper (Cu), or Mn. All
forms of water-soluble S are complexed with other
important nutrients (N, P, iron [Fe], Ca, K, Mg), so
careful consideration of other limiting nutrients in
a system must be made before deciding on which
form of S fertilizer to use. Gypsum (CaS04) is one
of the most common ways to add sulfur. It is also
possible to add S as single superphosphate if a combined application with P is desired. In order to minimize S leaching loss, applications of sulphate
should be made close to the time of peak plant
uptake.
The second main form of S fertilizer is elemental
sulfur (So), which is a water-insoluble solid. When
finely ground and mixed with soils, it is oxidized
to SO~- by microbes. The relative effectiveness of
So application is determined by the particle size
added, how and when it is added, the So oxidizing
characteristics of soil, and environmental conditions. Finer-size particles supply SO~- at a faster
rate. A mixture of So particle sizes is sometimes
desirable in order to achieve both rapid and longlasting S availability. Dissolution of SO~- is maximized if the pellets undergo wet-dry or freezethaw cycles. A way to maximize release of
plant-available S is to use irregularly shaped, porous granules. Alternatively, So can be applied with
bentonite, a clay that absorbs water and causes the
So granules to disintegrate.
The timing of additions is crucial, So must be
added as far ahead of germination and growth initiation as possible (Beaton et al. 1985). Even when
adding So with bentonite or porous granules, a few
months are needed for substantial release of plantavailable S. Elemental sulfur applications are more
efficient if they are incorporated into the soil. A
possible problem associated with So additions is
acidification of the soil, but this can be avoided by
making sure that the fertilizer is uniformly distributed (Beaton et al. 1985).
Isotopes
The use of isotopically labeled fertilizer allows
ecologists to ask more refined, mechanistic questions. Isotopes can serve as an invaluable tool to
check the efficiency of nutrient additions by tracking the fate of fertilizer additions, and thus helping
to determine if the lack of response of a system to
nutrient additions is mediated by low fertilizer
availability due to adsorption, loss, or microbial immobilization (Drury and Beauchamp 1991). Isotopes can also be used to look at the long-term fate
and availability of fertilizer (Preston and Mead
1994). Reciprocal transplants of litter from plots
fertilized with labeled and unlabeled nutrients can
help track recycling of fertilizer nutrients from the
litter (Power et al. 1986).
Isotopes can also elucidate the mechanisms of
ecosystem response to nutrient additions. Community composition changes may be explained by
differences in resource competition at different nutrient levels, which can be determined by comparing labeled fertilizer uptake by different species
(Nannipieri et al. 1985; Chang et al. 1996). Comparing amounts of labeled and unlabeled nutrients
in soil and plant pools can elucidate how nutrient
additions alter native soil nutrient availability (Pilbeam et al. 1997; Hart et al. 1986; Clinton and
Mead 1994).
Isotopes can be particularly useful for studying
system response to nutrient deposition. Isotopically
labeled nutrients can elucidate the pathways and
controls over nutrient movement through ecosystems, and can point to the mechanisms of ecosystem retention and loss (Nadelhoffer et al. 1995;
Buchmann et al. 1996; Koopmans et al. 1996).