ROLE
OF NEMATODE IN THE SOIL
The composition of nematode communities
(plant-parasitic and free-living) may be used as bio-indicators of soil health or
condition because composition correlates well with nitrogen cycling and
decomposition, two critical ecological processes in soil. Maturity and trophic
diversity indices withstand statistical rigor better than do abundances,
proportions, or ratios of trophic groups. Maturity indices respond to a variety
of land-management practices, based largely on inferred life history
characteristics of families. Similarity indices may be more useful than
diversity indices because they reflect taxon composition. Improving existing
indices or developing alternative indices refined by a greater understanding of
the biology of key taxa may enhance the utility of nematodes as bio indicators.
Nematodes
play an important role in essential soil processes. The direct contribution of
nematodes to nitrogen mineralization and distribution of biomass within plants
has been demonstrated in controlled experiments.In petri-dish experiments, more
nitrogen is available in the ammonium form when bacterivorous and fungivorous
nematodes are present than when they are absent (Trofymow and Coleman, 1982).
Nitrogen mineralized through microbial grazing is available subsequently to
plants (Seastedt et al., 1988; Sohlenius et al., 1988) and has been
demonstrated to affect biomass allocation in plants. In a microcosm experiment
with buffalo grass (Bouteloua gracilis), Ingham et al. (1985)
demonstrated that plant shoots grow larger in soils with bacteria, fungi, and
their respective grazers than in soils with less complex soil food webs. Root
biomass may also increase in the presence of microbial-grazing nematodes. Under
field conditions, bacterivorous and predatory nematodes are estimated to
contribute (directly and indirectly) about 8% to 19% of nitrogen mineralization
in conventional and integrated farming systems, respectively (Beare, 1997).
Nematodes contribute to nitrogen mineralization indirectly by grazing on
decomposer microbes, excreting ammonium, and immobilizing nitrogen in live
biomass (Beare, 1997; Ferris et al., 1998; Ingham et al., 1985). Predatory
nematodes also regulate nitrogen mineralization by feeding on microbial grazing
nematodes, a conduit by which resources pass from bottom to top trophic levels
(Wardle and Yeates, 1993). Although plants depend on nitrogen for their survival
and growth, ecological disruptions such as cultivation or additions of mineral
fertilizer increase nitrogen availability, sometimes in excess of, or
asynchronous with, plant needs. Increased availability of nitrate and ammonium
is associated inversely with successional maturity of nematode communities in
cultivated mineral soils for agricultural purposes (Neher, 2001).
FACTORS
THAT AFFECT THE DISTRIBUTION OF NEMATODES
Abiotic factors
§ Soil
pH: pH does not affect the nematodes directly. They can exist within normal pH
range (4-8) of agricultural soils, provided it safe for host plant growth.
§ Temperature:
Most plants parasitic nematodes have optimum thermal requirement between 15-300C.
Nematodes are able to survive a lower temperature limit of 50C and
upper limit 400C, but further extremes of lower and upper limit may
be lethal.
§ Soil
moisture and aeration: soils are beset with wide fluctuation in soil moisture.
Soil aeration is inversely related to moisture content and Nematodes require a
thin film of water for their biological activities. Excess moisture tends to
inhibit the locomotion of nematodes in soil.
§ Soil
texture: the relative proportion of sand, silt and clay determines the soil
texture. Certain Nematodes proves more pathogenic in light textured soil, Rotylechulus reniformis thrive well in
light textured soil.
§ Soil
chemicals: The concentrations of chemical constituents of soil fluctuate
drastically depending upon soil moisture content (irrigation, drought). The use
of chemical fertilizer, organic manures, pesticide etc further adds to the
dynamism of soil chemistry
Biotic
factors
§ Host
plants: crops and cropping pattern influence plant parasitic nematodes
populations tremendously. Some nematodes have wide host range while others
parasitize only selected host, the status of host is highly variable. The
multiplication rate of nematode species may be very high on a plant species
(good host) while some plant species (poor host) may not be very favourable for
multiplication
§ Microorganism
present in the soil: soil microorganism in the rhizosphere influence nematodes
in various ways. The nature of damage by nematodes to plants may be drastically
influenced by other pathogenic organisms (fungi, bacteria, and virus) present
in rhizosphere. Nematodes actively interact with these organisms resulting in
disease complexes which are common places in nature.
§ The
locomotion of nematodes per se is very limited, the can hardly move a few
centimetres in a year by their own movement. Short distance spread is usually facilitated
by irrigation, agricultural machinery, wind, livestock etc. long distance
occurs through plant propagating material, seedlings, saplings etc.
Reference
Neher D. A.
1999. Nematode communities in organically and conventionally
managed
agricultural soils. Journal of Nematology 31:142–154.
Moore, J. C.,
and P. C. de Ruiter. 1991. Temporal and spatial heterogeneity
of trophic
interactions within below-ground food webs.
Agriculture,
Ecosystems and Environment 34:371–397.
Role of Nematodes in Soil Health and Their Use as Indicators Journal of
Nematology 33(4):161–168. 2001.© The Society of Nematologists 2001.
