21. Biotic Manipulations Involving Belowground Animals
Verhoef and de Goede 1985; Bohlen et al. 1995).
The size of the barrier usually depends on the duration of the experiment and the size of the soil
samples to be removed during the experiment. Barriers (microplots) have been successfully used in
agriculture for analysis of soil pathogens of plants
(Barker et al. 1979; Noling and Ferris 1985) and in
ecology (enclosures) for excluding macrofauna
(Anderson et al. 1985; Verhoef and de Goede 1985;
Bohlen et al. 1995).
Litter bags are one of the primary experimental
tools to evaluate the relationship between the biota
and decomposition processes. These bags are usually made from a plastic-type window screen with
a known mesh size. Different sized mesh is used to
exclude various groups and sizes of animals. This
type of "semipermeable membrane" can exclude
organisms only in a hierarchical fashion (i.e., larger
and larger holes will allow in all but even larger
organisms, but not exclude smaller organisms) so
it does not allow for a full factorial manipulation.
For example, Tian et al. (1997), in a decomposition
study of a continuous maize!cassava cropping system, used O.5-mm mesh litter bags to allow access
of microarthropods and 0.08-mm mesh bags to exclude them. Measured amounts of organic matter
are placed in the bags for determining decomposition rates in ecosystem studies (Crossley and Hoglund 1962; Seastedt and Crossley 1980; Anderson
and Ingram 1993; Andren et al. 1995). Bags are left
on the soil surface or buried in the soil and retrieved
over a period of time to quantify the animals as well
as mass loss and nutrient status of the organic material. Santos and Whitford (1981) used litter bags
in conjunction with biocides to exclude selected
taxa.
Chemical Exclusions
Chemical or biocide manipulations were used for
many years by soil ecologists for several reasons,
including (1) they could easily be applied to soils
in most ecosystems, (2) they appeared to be selective for groups of soil organisms (e.g., nematicides
for nematodes, fungicides for fungi, insecticides for
microarthropods, etc.), and (3) they were replicable
over large-scale field plots.
However, in recent years (mid-1990 to present),
ecologists have largely decreased biocide use as a
means of soil biotic manipulation. This is because
321
of increasing evidence showing considerable variation in the reliability of the chemicals for the specific targets. The evidence includes (1) nontarget
effects on other biota, including humans (Freckman
and Caswell 1985; Ingham 1985; Colinas et al.
1994; Ingham et al. 1994; Coleman and Crossley
1996; Coleman et al. 1999), (2) independent effects
on other ecosystem parameters (NPP, root biomass), that are not listed by chemical companies,
(3) groundwater contamination (impacts on animals
living in the soil-water interface), and (4) degradation of the atmosphere. In addition, regulations
on the use of current biocides, replacement of
chemical formulations in formerly used biocides,
numerous new chemicals, and removal of chemicals from the market have made gathering information on the impact of each chemical on the soil,
the soil food web, and other ecosystem parameters
increasingly costly in time, as well as affecting our
ability to wisely interpret the results.
A synthesis of the numerous available chemicals
in the United States and elsewhere that are presently listed for fungi, insects, nematodes, and
bacteria, and documentation of their target and nontarget effects and their chemical degradation compounds is not within the scope of this chapter. We
caution those who might wish to use a biocide to
first check with authorities to see if the chemical is
still registered for legal use, and suggest a thorough
search in the soil ecology literature to identify nontarget effects and degradation products (see Ingham
1985; Colinas et al. 1994; Ingham et al. 1994).
The biocides differ considerably in their application and their effectiveness. The application
method for the biocide in the field is dependent on
the manufacturer's formulation; many are applied
as a liquid or granular to the soil and others as a
spray to the plant or soil. Some biocides are systemic and are translocated throughout the plant to
the roots with unknown effects on the rhizosphere
food web. The effectiveness of the biocide application for the target organism groups will vary with
soil physical and chemical properties, the amount
of rainfall, soil temperature, and the formulation of
the biocide (Thomlin and Gore 1974; Thomason et
al. 1983; Ingham 1985; Duncan and Noling 1998).
The side effect of biocides on nontarget organisms is the major reason that most soil ecologists
have chosen other types of manipulations. However, in the past, herbicides, insecticides, and
Verhoef and de Goede 1985; Bohlen et al. 1995).
The size of the barrier usually depends on the duration of the experiment and the size of the soil
samples to be removed during the experiment. Barriers (microplots) have been successfully used in
agriculture for analysis of soil pathogens of plants
(Barker et al. 1979; Noling and Ferris 1985) and in
ecology (enclosures) for excluding macrofauna
(Anderson et al. 1985; Verhoef and de Goede 1985;
Bohlen et al. 1995).
Litter bags are one of the primary experimental
tools to evaluate the relationship between the biota
and decomposition processes. These bags are usually made from a plastic-type window screen with
a known mesh size. Different sized mesh is used to
exclude various groups and sizes of animals. This
type of "semipermeable membrane" can exclude
organisms only in a hierarchical fashion (i.e., larger
and larger holes will allow in all but even larger
organisms, but not exclude smaller organisms) so
it does not allow for a full factorial manipulation.
For example, Tian et al. (1997), in a decomposition
study of a continuous maize!cassava cropping system, used O.5-mm mesh litter bags to allow access
of microarthropods and 0.08-mm mesh bags to exclude them. Measured amounts of organic matter
are placed in the bags for determining decomposition rates in ecosystem studies (Crossley and Hoglund 1962; Seastedt and Crossley 1980; Anderson
and Ingram 1993; Andren et al. 1995). Bags are left
on the soil surface or buried in the soil and retrieved
over a period of time to quantify the animals as well
as mass loss and nutrient status of the organic material. Santos and Whitford (1981) used litter bags
in conjunction with biocides to exclude selected
taxa.
Chemical Exclusions
Chemical or biocide manipulations were used for
many years by soil ecologists for several reasons,
including (1) they could easily be applied to soils
in most ecosystems, (2) they appeared to be selective for groups of soil organisms (e.g., nematicides
for nematodes, fungicides for fungi, insecticides for
microarthropods, etc.), and (3) they were replicable
over large-scale field plots.
However, in recent years (mid-1990 to present),
ecologists have largely decreased biocide use as a
means of soil biotic manipulation. This is because
321
of increasing evidence showing considerable variation in the reliability of the chemicals for the specific targets. The evidence includes (1) nontarget
effects on other biota, including humans (Freckman
and Caswell 1985; Ingham 1985; Colinas et al.
1994; Ingham et al. 1994; Coleman and Crossley
1996; Coleman et al. 1999), (2) independent effects
on other ecosystem parameters (NPP, root biomass), that are not listed by chemical companies,
(3) groundwater contamination (impacts on animals
living in the soil-water interface), and (4) degradation of the atmosphere. In addition, regulations
on the use of current biocides, replacement of
chemical formulations in formerly used biocides,
numerous new chemicals, and removal of chemicals from the market have made gathering information on the impact of each chemical on the soil,
the soil food web, and other ecosystem parameters
increasingly costly in time, as well as affecting our
ability to wisely interpret the results.
A synthesis of the numerous available chemicals
in the United States and elsewhere that are presently listed for fungi, insects, nematodes, and
bacteria, and documentation of their target and nontarget effects and their chemical degradation compounds is not within the scope of this chapter. We
caution those who might wish to use a biocide to
first check with authorities to see if the chemical is
still registered for legal use, and suggest a thorough
search in the soil ecology literature to identify nontarget effects and degradation products (see Ingham
1985; Colinas et al. 1994; Ingham et al. 1994).
The biocides differ considerably in their application and their effectiveness. The application
method for the biocide in the field is dependent on
the manufacturer's formulation; many are applied
as a liquid or granular to the soil and others as a
spray to the plant or soil. Some biocides are systemic and are translocated throughout the plant to
the roots with unknown effects on the rhizosphere
food web. The effectiveness of the biocide application for the target organism groups will vary with
soil physical and chemical properties, the amount
of rainfall, soil temperature, and the formulation of
the biocide (Thomlin and Gore 1974; Thomason et
al. 1983; Ingham 1985; Duncan and Noling 1998).
The side effect of biocides on nontarget organisms is the major reason that most soil ecologists
have chosen other types of manipulations. However, in the past, herbicides, insecticides, and