Agriculture Reference
In-Depth Information
Although adapted populations in areas with strongly seasonal climates have the capacity to
survive periods of drought, they nevertheless suffer heavy mortality, particularly among juveniles
unable to escape desiccation by moving deeper into the soil and becoming inactive (e.g., Gerard
1967). The severity and duration of summer droughts impose severe constraints on the duration of
earthworm activity and undoubtedly influence both the overall earthworm population density and
the biomass, which can be attained when growth and development are restricted to relatively short
periods in autumn and spring.
SOIL PROPERTIES
Many studies have attempted to relate earthworm distributions to a range of soil physical and
chemical parameters, often with inconclusive results. Other than soil moisture, the soil properties
that appear to be most important include texture, depth, pH, and organic matter content.
Medium-textured soils appear to be more favorable to earthworms than sandy soils or soils
with high clay content (Guild 1948). Nordstrm and Rundgren (1974) reported a positive relation-
ship between clay content and the abundance of
Aporrectodea caliginosa
,
Aporrectodea longa
,
Aporrectodea rosea
in 15 forest, pasture, and heath soils in Sweden, with
clay contents ranging from 5 to 25%. A stepwise multiple regression indicated a positive relationship
between clay content and the numbers and biomass of introduced
, and
Lumbricus terrestris
Aporrectodea
spp. in 113 pasture
soils in South Australia (Baker et al. 1992). Decreasing population densities of
were
linked with increasing proportions of sand and gravel in Egyptian soils (Khalaf El-Duweini and
Ghabbour 1965), and low earthworm densities (max 73 m
A. caliginosa
) occurred in sandy and silty coastal
grassland sites in County Wexford, Ireland, compared with those in similarly managed loam soils
(max 516 m
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) (Cotton and Curry 1980b). Although texture could have a direct effect on earthworm
activity in the case of abrasive gravelly soils, more often the influence of texture may be indirect
through its effect on moisture relationships. Heavy, poorly drained clay soils may become anaerobic
in areas of high rainfall, and light sandy soils are prone to drought.
The depth of soil is a significant factor governing earthworm distributions in temperate (Phil-
lipson et al. 1976) and tropical (Fragoso and Lavelle 1992; Lavelle et al. 1999) forest soils. Lack
of a sufficient depth of aerobic soil could be a factor limiting the establishment of deep-burrowing
earthworm species in soils reclaimed after mining (Curry and Cotton 1983).
Earthworms are generally absent from very acid soils (pH less than 3.5) and are scarce in soils
with pH less than 4.5. Although there are considerable differences among species in their pH
preference, the majority of temperate climate species are found in the pH range 5.0 to 7.4 (Satchell
1967; Bouch 1972). Other edaphic factors that have been linked with earthworm distributions
include calcium, magnesium, and nitrogen content (Fragoso and Lavelle 1992), and populations
can be affected adversely by high salt concentrations, which can occur, for example, in irrigated
soils (Khalaf El-Duweini and Ghabbour 1965).
The nature and quality of the soil organic matter are determined largely by the litter input from
the vegetation. Litter from grass, herbaceous plants, and deciduous trees growing on base-rich,
fertile soils is generally of high quality, with ratios of carbon to nitrogen close to or less than 20:1;
the vegetation on impoverished acidic soils produces tough, unpalatable litter low in nutrients
(carbon-to-nitrogen ratio more than 60:1) and unfavorable for earthworms. The organic matter that
provides the food base for the earthworm community is vitally important in determining their
distribution and abundance, and the soil organic matter content can sometimes be a good predictor
of earthworm abundance (Edwards and Bohlen 1996). For example, Hendrix et al. (1992) reported
a highly significant correlation between earthworm populations and soil organic carbon content
over a range of sites in the state of Georgia, which included a wide variety of soil and vegetation
types and management histories.
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