gut, ingested with soil or litter (Bertrand et al. 2015). Earthworms are considered to
be important ecological mediators capable of affecting soil functions and microbial
activity (Lipiec et al. 2016), generating an energy-rich mucus which triggers microorganisms by priming (Medina-Sauza et al. 2019) and signalling molecules with
hormone-like effects and influencing plant gene expression (Puga-Freitas and Blouin
2015).
The impact of earthworms on the microbial communities can be neutral, negative
or positive on the basis of species of earthworms. De Menezes et al. (2018) reported
the neutral effects of earthworms on soil bacterial communities, showing that the
introduction of the endogeic Aporrectodea trapezoid did not affect the microbial
communities of the entire soil. Hoeffner et al. (2018) observed the positive effects on
bacterial richness and diversity in the burrows as well as vermicompost of four
Lumbricus epi-anecic species compared to bulk soil. They had reported that bacterial
diversity increased in the final vermicompost as compared to the initial substrate. On
the other hand, negative effects of earthworms on bacterial diversity were observed
in earthworm casts by Koubová et al. (2015), which showed that the diversity of
bacterial species (estimated from cultural bacteria) decreased during the passage
through the intestine of the Eisenia fetida. This decline in microbial diversity after
soil intake was due to the increased dominance of several bacterial groups in
earthworm casts (Furlong et al. 2002; Gopal et al. 2017). More studies have
shown that the type of food eaten by earthworms seems to have little effect on the
diversity of bacterial communities in casts (Medina-Sauza et al. 2019).
19.3.2 Development of Soil Structure and Fertility
The structure of the soil is a critical factor in most soil functions, including soil
fertility (Bertrand et al. 2015). The interaction of different physical forces such as
soil water content, surrounding vegetation, presence or absence of organic matter,
soil tillage and soil microorganisms form the structure of soil (Sharma et al. 2017).
Among the soil organisms, earthworms have a prominent effect on the soil structure
due to the reorganization of soil as it feeds and moves in the soil (Martin and
Marinissen 1993). They also affect the mechanical and hydraulic properties of soil
through their burrowing activities which produce macropores that have a significant
impact on water absorption and are therefore important for water-supplying crops
and controlling surface runoff and erosion (Bertrand et al. 2015). Earthworms also
improve the soil structure by forming soil aggregates which help in soil erosion
control (Julka 2008). A large proportion of soil passes through the guts of earthworm, as they turn over the top 15 cm of soil in 10–20 years. Earthworms contribute
to soil organic matter dynamics in agroecosystems by consuming an estimated
2–15 mg/ha/year of organic residues and process as much as 10% of the topsoil
each year (Whalen and Parmelee 2000; Topoliantz et al. 2000). Earthworms are
estimated to produce casts of approximately 40–100 tons/ha/year and thus contribute
19 Earthworm Communities and Soil Structural Properties
337
be important ecological mediators capable of affecting soil functions and microbial
activity (Lipiec et al. 2016), generating an energy-rich mucus which triggers microorganisms by priming (Medina-Sauza et al. 2019) and signalling molecules with
hormone-like effects and influencing plant gene expression (Puga-Freitas and Blouin
2015).
The impact of earthworms on the microbial communities can be neutral, negative
or positive on the basis of species of earthworms. De Menezes et al. (2018) reported
the neutral effects of earthworms on soil bacterial communities, showing that the
introduction of the endogeic Aporrectodea trapezoid did not affect the microbial
communities of the entire soil. Hoeffner et al. (2018) observed the positive effects on
bacterial richness and diversity in the burrows as well as vermicompost of four
Lumbricus epi-anecic species compared to bulk soil. They had reported that bacterial
diversity increased in the final vermicompost as compared to the initial substrate. On
the other hand, negative effects of earthworms on bacterial diversity were observed
in earthworm casts by Koubová et al. (2015), which showed that the diversity of
bacterial species (estimated from cultural bacteria) decreased during the passage
through the intestine of the Eisenia fetida. This decline in microbial diversity after
soil intake was due to the increased dominance of several bacterial groups in
earthworm casts (Furlong et al. 2002; Gopal et al. 2017). More studies have
shown that the type of food eaten by earthworms seems to have little effect on the
diversity of bacterial communities in casts (Medina-Sauza et al. 2019).
19.3.2 Development of Soil Structure and Fertility
The structure of the soil is a critical factor in most soil functions, including soil
fertility (Bertrand et al. 2015). The interaction of different physical forces such as
soil water content, surrounding vegetation, presence or absence of organic matter,
soil tillage and soil microorganisms form the structure of soil (Sharma et al. 2017).
Among the soil organisms, earthworms have a prominent effect on the soil structure
due to the reorganization of soil as it feeds and moves in the soil (Martin and
Marinissen 1993). They also affect the mechanical and hydraulic properties of soil
through their burrowing activities which produce macropores that have a significant
impact on water absorption and are therefore important for water-supplying crops
and controlling surface runoff and erosion (Bertrand et al. 2015). Earthworms also
improve the soil structure by forming soil aggregates which help in soil erosion
control (Julka 2008). A large proportion of soil passes through the guts of earthworm, as they turn over the top 15 cm of soil in 10–20 years. Earthworms contribute
to soil organic matter dynamics in agroecosystems by consuming an estimated
2–15 mg/ha/year of organic residues and process as much as 10% of the topsoil
each year (Whalen and Parmelee 2000; Topoliantz et al. 2000). Earthworms are
estimated to produce casts of approximately 40–100 tons/ha/year and thus contribute
19 Earthworm Communities and Soil Structural Properties
337