320
tion by transporting microbes and microinvertebrates, and by rearranging or chemically altering
soil such that its properties are substantially different from unmanipulated soil. The approaches used
in investigations of the effect of soil biota on ecosystem processes should consider direct and indirect effects, although the latter are much more difficult to test.
Exclusions as Biotic Manipulations
Species removals or exclusions have been used as
biotic manipulations more often than species introductions. This is because (1) the species introduced
could become invasive and (2) plant pathology research over the past 100 years has shown problems
in establishing a beneficial species into a soil habitat. In soil experiments, exclusion generally refers
to exclusion of a trophic group, a high-level taxonomic grouping such as an order or phylum (which
could include many trophic groups), or animals of
a certain size range. Removal of an organism at the
species level from a soil is very difficult.
The exclusion of soil biota theoretically involves
two steps, the actual removal of the target taxon
and a means to keep it out. In soils, exclusion from
a plot of land works better for vertebrates than invertebrates that are highly mobile (ants, termites,
earthworms), or transferred by rain or wind (nematodes, mites, protozoa), or that have survival mechanisms that allow them to withstand any chemical
exclusion. The physical removal of a soil organism
is only effective where individuals are large (visible) and occur in low densities. For example, small
mammals can be trapped and removed (Reichman
and Smith 1985; Huntly and Inouye 1988). However, in the field it is almost impossible to remove
all individual invertebrates from the soil by any of
the methods we discuss. Therefore, for soil invertebrates and microflora, different types of exclusion
methods, for example, litter bags and electroshocking soil, are often combined to assure the absence
of animals (Seastedt et al. 1987; Couteaux et al.
1991). We urge caution in the selection of methods
for removal of invertebrates, since the manipulation
can affect other factors (e.g., soil physical and
chemical properties) (Huhta et al. 1989; Coleman
et al. 1999).
Diana H. Wall and O. James Reichman
Physical Exclusion Methods
Invertebrate biota are removed by two methodsphysical and chemical-and have been the subject
of many reviews (Seastedt 1984; Freckman et al.
1986; Huhta et al. 1989; Coleman and Crossley
1996). The physical methods include barriers, litter
bags (Crossley and Hoglund 1962; Coleman and
Crossley 1996), autoclaving/steam sterilizing (Ingham et al. 1985), microwave (Ferris 1984; Huhta
et al. 1989), liquid nitrogen (Sulkava et al. 1996),
drying (Huhta et al. 1989), gamma irradiation
(Griffiths 1987), and electroshocking for removing
earthworms (Bohlen et al. 1995; Blair et al. 1996).
Methods such as barriers, litter bags and electroshocking are used in the field to selectively exclude
larger invertebrates (Blair et al. 1996; Coleman et
al. 1999). Autoclaving, steam sterilizing, microwaving, freezing, drying, and gamma irradiation
are all used to exclude as many groups of taxa as
possible from soil removed from the field. Once the
soil is removed from the experimental plot, it is
taken to the laboratory, the method to remove the
biota applied, and the soil reintroduced back into
the field. The type of soil, the volume of soil, and
the variation in the intensity and duration of the
treatment all interact to determine the taxa that are
removed. It is difficult, even with gamma irradiation, to remove all the soil organisms.
If the experiment has a temporal component, it
is necessary to keep the target organisms initially
excluded out of the plot. This is often achieved for
vertebrates and invertebrates with physical barriers
such as "fences" and trenches. Fences are small and
narrow, generally made out of plastic or metal
flashing that is driven into the soil to a depth sufficient to leave several centimeters remaining
aboveground. Barriers for invertebrates, also
termed microplots, cylinders, or enclosures, are
"fences" composed of PVC, concrete, fiberglass, or
another material. Essentially, these barriers prevent
animals from moving into the experimental plot.
Soil within the barriers is either left as is, or the
target animals are removed from the soil by an additional exclusion method, such as soil sterilization,
gamma irradiation, or others listed previously. Barriers can be circular, ranging from about 5 cm to
80 cm in diameter and 0.5 m in depth, or rectangular and can be inserted to a depth of 1 to 2 m
depending on the soil type (Anderson et al. 1985;
tion by transporting microbes and microinvertebrates, and by rearranging or chemically altering
soil such that its properties are substantially different from unmanipulated soil. The approaches used
in investigations of the effect of soil biota on ecosystem processes should consider direct and indirect effects, although the latter are much more difficult to test.
Exclusions as Biotic Manipulations
Species removals or exclusions have been used as
biotic manipulations more often than species introductions. This is because (1) the species introduced
could become invasive and (2) plant pathology research over the past 100 years has shown problems
in establishing a beneficial species into a soil habitat. In soil experiments, exclusion generally refers
to exclusion of a trophic group, a high-level taxonomic grouping such as an order or phylum (which
could include many trophic groups), or animals of
a certain size range. Removal of an organism at the
species level from a soil is very difficult.
The exclusion of soil biota theoretically involves
two steps, the actual removal of the target taxon
and a means to keep it out. In soils, exclusion from
a plot of land works better for vertebrates than invertebrates that are highly mobile (ants, termites,
earthworms), or transferred by rain or wind (nematodes, mites, protozoa), or that have survival mechanisms that allow them to withstand any chemical
exclusion. The physical removal of a soil organism
is only effective where individuals are large (visible) and occur in low densities. For example, small
mammals can be trapped and removed (Reichman
and Smith 1985; Huntly and Inouye 1988). However, in the field it is almost impossible to remove
all individual invertebrates from the soil by any of
the methods we discuss. Therefore, for soil invertebrates and microflora, different types of exclusion
methods, for example, litter bags and electroshocking soil, are often combined to assure the absence
of animals (Seastedt et al. 1987; Couteaux et al.
1991). We urge caution in the selection of methods
for removal of invertebrates, since the manipulation
can affect other factors (e.g., soil physical and
chemical properties) (Huhta et al. 1989; Coleman
et al. 1999).
Diana H. Wall and O. James Reichman
Physical Exclusion Methods
Invertebrate biota are removed by two methodsphysical and chemical-and have been the subject
of many reviews (Seastedt 1984; Freckman et al.
1986; Huhta et al. 1989; Coleman and Crossley
1996). The physical methods include barriers, litter
bags (Crossley and Hoglund 1962; Coleman and
Crossley 1996), autoclaving/steam sterilizing (Ingham et al. 1985), microwave (Ferris 1984; Huhta
et al. 1989), liquid nitrogen (Sulkava et al. 1996),
drying (Huhta et al. 1989), gamma irradiation
(Griffiths 1987), and electroshocking for removing
earthworms (Bohlen et al. 1995; Blair et al. 1996).
Methods such as barriers, litter bags and electroshocking are used in the field to selectively exclude
larger invertebrates (Blair et al. 1996; Coleman et
al. 1999). Autoclaving, steam sterilizing, microwaving, freezing, drying, and gamma irradiation
are all used to exclude as many groups of taxa as
possible from soil removed from the field. Once the
soil is removed from the experimental plot, it is
taken to the laboratory, the method to remove the
biota applied, and the soil reintroduced back into
the field. The type of soil, the volume of soil, and
the variation in the intensity and duration of the
treatment all interact to determine the taxa that are
removed. It is difficult, even with gamma irradiation, to remove all the soil organisms.
If the experiment has a temporal component, it
is necessary to keep the target organisms initially
excluded out of the plot. This is often achieved for
vertebrates and invertebrates with physical barriers
such as "fences" and trenches. Fences are small and
narrow, generally made out of plastic or metal
flashing that is driven into the soil to a depth sufficient to leave several centimeters remaining
aboveground. Barriers for invertebrates, also
termed microplots, cylinders, or enclosures, are
"fences" composed of PVC, concrete, fiberglass, or
another material. Essentially, these barriers prevent
animals from moving into the experimental plot.
Soil within the barriers is either left as is, or the
target animals are removed from the soil by an additional exclusion method, such as soil sterilization,
gamma irradiation, or others listed previously. Barriers can be circular, ranging from about 5 cm to
80 cm in diameter and 0.5 m in depth, or rectangular and can be inserted to a depth of 1 to 2 m
depending on the soil type (Anderson et al. 1985;