Agriculture Reference
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on growing root-tips. Selective reproduction in suitable environments is an additional
mechanism which favours the colonisation of soils (review by Yeates, 1981).
Density and biomass of populations
Nematodes have a remarkable ability to resist environmental stress. As a result, certain
community parameters, e.g., species diversity, density and biomass tend to increase along
a gradient of increasing environmental stress. This trend slows and later reverses in envi-
ronments with particularly high stress levels (Procter, 1990).
Most estimates of nematode populations are in the range of individuals
with a corresponding fresh biomass of 1-10 g (Sohlenius, 1980; Petersen and Luxton,
1982). Densities up to 30-50 ind. have been recorded in such different environ-
ments as temperate grassland in France (Ricou, 1978), a meadow-steppe in Russia
(Ghilarov and Chernov, 1974) and an oak-ash forest with a mull-type soil in Gemany
(Volz, 1951).
Relatively high densities have been recorded in such extreme environments as
the high arctic tundra at Devon Island
and the Tajmyr Peninsula in the
North of Siberia In sub-desertic areas of the USA (1.2 in Nevada) and
USSR (1.1 to 6 ) intermediate values have been reported (Chernov, 1975;
Freckman et al., 1975; Procter, 1977; Ryan, 1977).
Population densities less than ind. have only been recorded in desert soils
(Steinberger et al., 1988), extreme tundra soils (Point Barrow, Alaska) and some tropical
forests of Puerto Rico, Kenya and Uganda (Coleman, 1970; Kitazawa, 1971). In the
latter case however, the method used (centrifugation-flotation) is suspected to have given
underestimates and the densities of 1.5-1.9 reported for tropical forests of the
Congo Basin using classical methods (modified Baerman technique) (Maldague, 1961)
are considered more representative of that type of ecosystem (Petersen and Luxton,
1982). In New Caledonian tropical forests with ultrabasic soils, Yeates (1991) reported
densities of 95,000 to 265,000 and suggested that nematode populations in tropical
forests typically occur at the scale of hundreds of thousands compared with the
several millions found in temperate deciduous forests and coniferous forests. In the
tropical montane grasslands of Montagne Pelée (Martinique), Cadet and Van den Berg
(1992) reported population densities of 1.8
The main variables influencing abundance are the soil hydrological regimes and
the relative accumulation of organic matter on the surface. Populations are more
numerous in 'mesic' than wet soils with the lowest densities being found in 'dry' soils.
Abundances, and particularly biomasses, are greater in grasslands than in forests (due
to an increase of phytoparasitic populations), and in mineral mull-type soils than in
moders or acid mors with large surficial accumulations of organic matter (Petersen
and Luxton, 1982).
Finally, in grasslands in the USA, Yeates and Coleman (1982) observed positive
correlations between the abundance of nematodes and root biomass and above-ground
production. Such a relationship appears to be general in grasslands whereas, in forest
soils, contrasting observations, have been made.
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