The Soil Biology Primer
Chapter 6: NEMATODES
By Elaine R. Ingham
THE LIVING SOIL: NEMATODES
Nematodes are non-segmented worms typically 1/500 of an inch (50 µm) in
diameter and 1/20 of an inch (1 mm) in length. Those few species
responsible for plant diseases have received a lot of attention, but far
less is known about the majority of the nematode community that plays
beneficial roles in soil.
An incredible variety of nematodes function at several trophic levels
of the soil food web. Some feed on the plants and algae (first trophic
level); others are grazers that feed on bacteria and fungi (second trophic
level); and some feed on other nematodes (higher trophic levels).
Free-living nematodes can be divided into four broad groups based on
their diet. Bacterial-feeders consume bacteria.
Fungal-feeders feed by puncturing the cell wall of fungi and
sucking out the internal contents. Predatory nematodes eat all
types of nematodes and protozoa. They eat smaller organisms whole, or
attach themselves to the cuticle of larger nematodes, scraping away until
the prey’s internal body parts can be extracted. Omnivores eat a
variety of organisms or may have a different diet at each life stage.
Root-feeders are plant parasites, and thus are not free-living in
|Figure 1: Most nematodes in the
soil are not plant parasites. Beneficial nematodes help control
disease and cycle nutrients.
|Figure 2: A predatory nematode consumes a
Credit: Kathy Merrifield,
Oregon State University, Corvallis.
WHAT DO NEMATODES DO?
Nutrient cycling. Like protozoa, nematodes are
important in mineralizing, or releasing, nutrients in plant-available
forms. When nematodes eat bacteria or fungi, ammonium (NH4+) is released
because bacteria and fungi contain much more nitrogen than the nematodes
Grazing. At low nematode densities, feeding by
nematodes stimulates the growth rate of prey populations. That is,
bacterial-feeders stimulate bacterial growth, plant-feeders stimulate
plant growth, and so on. At higher densities, nematodes will reduce the
population of their prey. This may decrease plant productivity, may
negatively impact mycorrhizal fungi, and can reduce decomposition and
immobilization rates by bacteria and fungi. Predatory nematodes may
regulate populations of bacterial-and fungal-feeding nematodes, thus
preventing over-grazing by those groups. Nematode grazing may control the
balance between bacteria and fungi, and the species composition of the
Dispersal of microbes. Nematodes help distribute
bacteria and fungi through the soil and along roots by carrying live and
dormant microbes on their surfaces and in their digestive systems.
Food source. Nematodes are food for higher level
predators, including predatory nematodes, soil microarthropods, and soil
insects. They are also parasitized by bacteria and fungi.
Disease suppression and development. Some nematodes
cause disease. Others consume disease-causing organisms, such as
root-feeding nematodes, or prevent their access to roots. These may be
potential biocontrol agents.
|Figure 3: Fungal-feeding nematodes have
small, narrow stylets, or spears, in their stoma (mouth) which they
use to puncture thecell walls of fungal hyphae and withdraw the cell
fluid. This interaction releases plant-available nitrogen from
Credit: Elaine R.
|Figure 4: This bacterial-feeding
nematode, Elaphonema, has ornate lip structures that
distinguish it from other nematodes. Bacterial-feeders release
plant-available nitrogen when they consume
Credit: Elaine R.
|Figure 5: The Pratylenchus, or
lesion nematode, has a shorter, thicker stylet in its mouth than the
root feeder in Figure 6.
Merrifield, Oregon State University, Corvallis
|Figure 6: Root-feeding nematodes use
their stylets to puncture the thick cell wall of plant root cells
and siphon off the internal contents of the plant cell. This usually
causes economically significant damage to crops. The curved stylet
seen inside this nematode is characteristic of the genus
Credit: Elaine R.
WHERE ARE NEMATODES?
Nematodes are concentrated near their prey groups. Bacterial-feeders
abound near roots where bacteria congregate; fungal-feeders are near
fungal biomass; root-feeders are concentrated around roots of stressed or
susceptible plants. Predatory nematodes are more likely to be abundant in
soils with high numbers of nematodes.
Because of their size, nematodes tend to be more common in
coarser-textured soils. Nematodes move in water films in large (>1/500
inch or 50 µm) pore spaces.
Agricultural soils generally support less than 100 nematodes in each
teaspoon (dry gram) of soil. Grasslands may contain 50 to 500 nematodes,
and forest soils generally hold several hundred per teaspoon. The
proportion of bacterial-feeding and fungal-feeding nematodes is related to
the amount of bacteria and fungi in the soil. Commonly, less disturbed
soils contain more predatory nematodes, suggesting that predatory
nematodes are highly sensitive to a wide range of disturbances.
NEMATODES AND SOIL QUALITY
Nematodes may be useful indicators of soil quality because of their
tremendous diversity and their participation in many functions at
different levels of the soil food web. Several researchers have proposed
approaches to assessing the status of soil quality by counting the number
of nematodes in different families or trophic groups.* In addition to
their diversity, nematodes may be useful indicators because their
populations are relatively stable in response to changes in moisture and
temperature (in contrast to bacteria), yet nematode populations respond to
land management changes in predictable ways. Because they are quite small
and live in water films, changes in nematode populations reflect changes
in soil microenvironments.
*Blair, J. M. et al. 1996. Soil invertebrates as indicators of soil
quality. In Methods for Assessing Soil Quality, SSSA Special
Publication 49, pp. 273-291.
BUG BIOGRAPHY: Nematode Trappers
One group of fungi may be a useful biological control agent against
parasitic nematodes. These predatory fungi grow through the soil, setting
out traps when they detect signs of their prey. Some species use sticky
traps, others make circular rings of hyphae to constrict their prey. When
the trap is set, the fungi put out a lure, attracting nematodes that are
looking for lunch. The nematode, however, becomes lunch for the
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