In Patra (1977) and Kale and Krishnamoorthy (1978, 1982)

In
subtropical India main activity of earthworms was during rainy season and
reached at peak in September and October and minimum density in May – June
(Gates, 1961; Dash and Patra, 1977; Dash and Senapati, 1980; Chauhan, 1980;
Choudhuri and Mitra, 1983; Mishra and Dash, 1984). Increase in population of Aporrectodea caliginosa was reported by
El- Duweini and Ghabbour (1965a) with rise of soil moisture from 15% to 34%.
The main activity of Hyperiodrilus
africanus in Nigeria was reported to be in the beginning of wet season in
soils with moisture levels of 12.5% to 17.2% (Madge, 1969). Reynolds in 1972c
found that the density and biomass of earthworms declined with the decrease in
moisture of soil. Due to extreme draught a surface dwelling West African species,
Millsonia anomala became dormant and
again recovered at 8% to10% of soil moisture (Lavelle et al., 1974). Due to
decline in moisture earthworms often migrated to deeper layers (Edward and
Lofty, 1977). Dash and Patra (1977) and Kale and Krishnamoorthy (1978, 1982) have recorded maximum number of
earthworms and biomass in the rainy and late rainy period. According to
Dash and Senapati (1980) earthworms became dormant when the soil moisture fell
down to 10% or below in Orissa.

Seasonal
changes in population of three species of earthworms was studied in Bangalore
by Kale and Krishnamoorthy (1982) and found that the population of adult Lampito mauritii increased with a total
rainfall but more important were the trophic factors. Fragoso and Lavelle
(1987, 1992) and Fragoso (1993) observed a relationship between earthworm
activity and rainfall; during the rainy season, the worms moved toward the
upper layers of soil and in the dry season they migrated to lower layers.
Significant positive correlation was obtained between seasonal changes in the
soil moisture and earthworm biomass (Julka and Paliwal, 1990). Ismail et al.
(1990) and Ghosh (1993) reported positive relationship between soil moisture
and population density of overall macro invertebrates including earthworms.
Julka and Paliwal (1990) observed a sharp decline in the juvenile population of
Octolasion tyrtaeum in an orchard
with low soil moisture (10%) in the western Himalayas. Soil moisture played a
major role in the distribution and occurrence of various earthworm species
(Bohlen et al., 1995; Edwards and Bohlen, 1996; Auerswald et al., 1996).
Earthworms’ population density was the result of the interaction of a number of
factors of which moisture is of greater importance (Valle et al., 1997). Biomass and species richness of soil
invertebrates were influenced by soil moisture (Marra and Edmonds, 1998).

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Jimenez
and Decaens (2000) investigated the vertical distribution of native earthworm
species in natural and disturbed level fields with small patchy vegetations.
They found that small sized mature worms of the Ocnerodrilidae migrated to
deeper layers in dry season. Moisture of the soil had an important effect on
the vertical distribution of earthworms. Schmidt and Curry (2001) reported low
levels of earthworm population in soils with low moisture, and Smetak et al.
(2007) opined that earthworms were sensitive to moisture stress. Sinha and
Srivastava (2001) studied the effect of seasonal variation on density and
biomass of population growth of the earthworms and noticed that the rate of
population growth was maximum in July and minimum in June when environment was
unfavorable in terms of moisture. Bisht et al. (2003) found more clitellate
earthworms during the wet period in central Himalayan Tarai region. Martinez
(2006) also reported abundance of earthworms in upper layers of soil during the
rainy season and their migration to lower layers in dry season. Significant
positive correlation was established between soil moisture and earthworm.
Chaudhuri et al. (2008) studied the community structure of earthworms in rubber
plantations and their adjacent mixed forests in Tripura and discovered that
earthworms showed wide range of tolerance to soil moisture (8.7%-51%).

Joshi
and Aga (2009) noted increased earthworm activity in high rainfall areas. In
North-west and Central Kashmir, Najar and Khan (2011) correlated higher species
diversity of earthworms with high moisture content of soil.  The density of earthworm increased along with
parameters such as rainfall and soil moisture in agroforestry based
experimental plots in Mizoram (Lalthanzara et al., 2011). Study on earthworm
density and biomass at different land use types in central Himalaya, found that
all the species except Bimastos parvus
showed a strong effect of season on population with highest values during rainy
season (Bhadauria et al., 2012). Mariappan et al. (2013) also reported that
during rainy season earthworm population density was higher as compared to low
population density during summer and winter. Kalu et al. (2015) found a
significant positive correlation between earthworm population density and soil
moisture content.

2.8.4. Soil pH

The
pH changes in soil effected the distribution, density and population of
earthworms. Wherry (1924) was probably the first to record the effect of soil
reaction on the life of earthworms. He recorded that slightly acidic and
neutral soils suited these organisms, which could not tolerate pH lower than
6.0. Several workers found that earthworms preferred neutral soil (Arrhenius
1921; Moore, 1922; Salisbury, 1925; Allee et al., 1930; Bodenheimer, 1935;
Petrov, 1946). Olson (1928) recorded a preference range of 5.4 to 8.6 for
earthworms from Ohio. Evans and Guild (1947) gave a pH range of 6.3 to 8.1 for
habitats of various European earthworms.

Based
on their occurrence in soils with different values of pH, Satchell (1955)
classified the European lumbricids into three different categories: i) Acid
tolerant: Bimastos eiseni, Dendrobaena octaedra and Dendrobaena rubida;  ii) Acid intolerant: Aporrectodea caliginosa, Allolobophora
nocturna, Allolobophora chlorotica,
Allolobophora longa, and Allolobophora rosea;  iii) Ubiquitous species: Lumbricus castaneus, Lumbricus
rubellus, Lumbricus terrestris, Octolasion cyaneum and Dendrobaena subrubicunda. 

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