21. Biotic Manipulations Involving Belowground Animals
cid earthwonns, offers an opportunity to examine
changes in nutrient retention in a number of ecosystems (Boag et al. 1998). Introduction of exotic
soil animals that are plant pathogenic or parasitic
can be especially problematic but offer opportunities for study of a single species on plant productivity in diverse ecosystems. The introduction from
the colder Gennan soils of a plant parasite, the
sugar beet cyst nematode, Heterodera schactii, into
the desert agricultural soils of California, resulted
in a fast spread of this soil species, an adverse economic impact to sugar beet industry due to loss of
NPP and root biomass, and enactment of quarantine
procedures to prevent spread (Miller 1986). More
recently, the plant parasitic nematode, Belonolaimus longicaudatus, has been introduced from soils
of the southeastern United States to California,
again causing detrimental impacts to plants and
economic costs (Cherry et al. 1997).
Habitat Manipulations
Manipulations of habitat can involve altering the
physical structure of the soil, transplanting soil and!
or its inhabitants, excluding organisms, or excluding certain nutrients or water (Seastedt et al. 1987).
Physical Alteration
Alteration of the soil by using various tilling regimes often generates major findings about soil
functional groups and ecosystem processes (Robertson et al. 1993; Robertson and Freckman 1995),
as well as the impact on soil biodiversity (Swift and
Anderson 1994; Heal et al. 1996). Plowing affects
several features of the soil, including its friability,
water infiltration, and nitrification (usually by
breaking up soil aggregates) (Paustian et al. 1997).
Manipulations can take place across entire fields or
as plots within a field. Comparisons between the
functional groups or species in nonaltered and altered soils and the rate of the ecosystem process
provide a measure of the role of the organism.
The effects of the biota can also be examined by
experimentally altering soil. For example, pocket
gophers excavate extensive burrows belowground
and deposit the tailings and nutrients on the soil
surface as mounds. Experimentally, both the burrows and the mounds can be simulated by hand
323
excavating and by piling soil of the appropriate type
on the surface. Even the spatial pattern of such disturbances can be altered to evaluate the physical
impact of the disturbance and how the pattern itself
influences ecosystem processes (Reichman et al.
1993).
Experimental manipulation of fire regimes is often used to study the effects of burning on aboveground processes. However, fire also affects the soil
and its inhabitants (Seastedt 1988; Seastedt and
Reddy 1991; Blair 1997) by altering the temperature immediately (as the fire passes) or in the near
tenn (by yielding dark ash, which absorbs light energy and insulates the soil). Furthennore, as a second order effect, the resulting plant community also
alters the subsequent below ground community and
its effect on ecosystem processes (Ojima et al.
1990). Following the fire, measuring the rate oflitter decompositon (litter bags) and the response of
the soil biota in the fire-manipulated versus unmanipulated soils provides infonnation on the resilience and resistance of the system, and the redundancy of the soil food web.
Mechanical manipulations, such as soil warming,
the addition or exclusion of water, and increasing
carbon dioxide (C0 2 ) can be employed (Whitford
et al. 1981; Freckman et al. 1987; Couteaux et al.
1991; Treonis and Lussenhop 1997).
Introductions and Transplants
One of the most effective types of manipulations
involves reciprocal transplants of some organism or
habitat feature of interest. For example, soil cores
of similar sizes can be extracted from one location
and transposed with cores from another area to determine the influence of native soil on the biotic
and abiotic characteristics of each soil type (J. Blair,
personal communication). In a subset of this type
of transplant, soil may be extracted and sterilized
and reintroduced into the same hole. Transplants
are an especially effective type of manipulation if
care is taken to treat all treatments similarly. Thus,
in the reciprocal core transplant manipulation, controls that include simulated transplants within the
same soil type must be used (i.e., core extracted and
put back into the same hole).
Transplanting soil and plants can also be useful
for determining the effect of aboveground vegetation on below ground processes. This happens on a
cid earthwonns, offers an opportunity to examine
changes in nutrient retention in a number of ecosystems (Boag et al. 1998). Introduction of exotic
soil animals that are plant pathogenic or parasitic
can be especially problematic but offer opportunities for study of a single species on plant productivity in diverse ecosystems. The introduction from
the colder Gennan soils of a plant parasite, the
sugar beet cyst nematode, Heterodera schactii, into
the desert agricultural soils of California, resulted
in a fast spread of this soil species, an adverse economic impact to sugar beet industry due to loss of
NPP and root biomass, and enactment of quarantine
procedures to prevent spread (Miller 1986). More
recently, the plant parasitic nematode, Belonolaimus longicaudatus, has been introduced from soils
of the southeastern United States to California,
again causing detrimental impacts to plants and
economic costs (Cherry et al. 1997).
Habitat Manipulations
Manipulations of habitat can involve altering the
physical structure of the soil, transplanting soil and!
or its inhabitants, excluding organisms, or excluding certain nutrients or water (Seastedt et al. 1987).
Physical Alteration
Alteration of the soil by using various tilling regimes often generates major findings about soil
functional groups and ecosystem processes (Robertson et al. 1993; Robertson and Freckman 1995),
as well as the impact on soil biodiversity (Swift and
Anderson 1994; Heal et al. 1996). Plowing affects
several features of the soil, including its friability,
water infiltration, and nitrification (usually by
breaking up soil aggregates) (Paustian et al. 1997).
Manipulations can take place across entire fields or
as plots within a field. Comparisons between the
functional groups or species in nonaltered and altered soils and the rate of the ecosystem process
provide a measure of the role of the organism.
The effects of the biota can also be examined by
experimentally altering soil. For example, pocket
gophers excavate extensive burrows belowground
and deposit the tailings and nutrients on the soil
surface as mounds. Experimentally, both the burrows and the mounds can be simulated by hand
323
excavating and by piling soil of the appropriate type
on the surface. Even the spatial pattern of such disturbances can be altered to evaluate the physical
impact of the disturbance and how the pattern itself
influences ecosystem processes (Reichman et al.
1993).
Experimental manipulation of fire regimes is often used to study the effects of burning on aboveground processes. However, fire also affects the soil
and its inhabitants (Seastedt 1988; Seastedt and
Reddy 1991; Blair 1997) by altering the temperature immediately (as the fire passes) or in the near
tenn (by yielding dark ash, which absorbs light energy and insulates the soil). Furthennore, as a second order effect, the resulting plant community also
alters the subsequent below ground community and
its effect on ecosystem processes (Ojima et al.
1990). Following the fire, measuring the rate oflitter decompositon (litter bags) and the response of
the soil biota in the fire-manipulated versus unmanipulated soils provides infonnation on the resilience and resistance of the system, and the redundancy of the soil food web.
Mechanical manipulations, such as soil warming,
the addition or exclusion of water, and increasing
carbon dioxide (C0 2 ) can be employed (Whitford
et al. 1981; Freckman et al. 1987; Couteaux et al.
1991; Treonis and Lussenhop 1997).
Introductions and Transplants
One of the most effective types of manipulations
involves reciprocal transplants of some organism or
habitat feature of interest. For example, soil cores
of similar sizes can be extracted from one location
and transposed with cores from another area to determine the influence of native soil on the biotic
and abiotic characteristics of each soil type (J. Blair,
personal communication). In a subset of this type
of transplant, soil may be extracted and sterilized
and reintroduced into the same hole. Transplants
are an especially effective type of manipulation if
care is taken to treat all treatments similarly. Thus,
in the reciprocal core transplant manipulation, controls that include simulated transplants within the
same soil type must be used (i.e., core extracted and
put back into the same hole).
Transplanting soil and plants can also be useful
for determining the effect of aboveground vegetation on below ground processes. This happens on a