Agriculture Reference
In-Depth Information
TABLE 2.1
The Seven Main Mechanisms by Which Earthworms Affect Plant (Mostly Root) Growth
either Directly or Indirectly through Physicochemical or Biological Changes to the Soil
Environment
Mechanism Category (Type)
Mechanism Mode
Biological
Physical
Chemical
Indirect
(mediated
through changes in
the rooting
environment, or via
interactions with
organisms that affect
root growth and
production)
1. Dispersal or changes in populations
and activity of beneficial
microorganisms
6. Changes in soil
structure caused by
earthworms
7. Changes in nutrient
spatiotemperal
availability caused by
earthworms
(plant growth
promoting rhizobacteria, N
(pore and
aggregate size distribution
and associated processes,
including aeration, water
retention, hydraulic
conductivity, infiltration,
erosion, runnoff, aggregate
and crust formation and
breakdown,
compaction/soil slumping
and decompaction/soil
loosening)
fixing root
symbionts, saprophytic and mycorrhizal
fungi, microbial biocontrol agents,
microbivorous and entomopathogenic
nematodes, protozoa)
(release or
immobilization of
different plant nutrients,
leaching, denitrification,
volatilization, OM
mineralization, protection
and/or humification,
chelation of metals, pH
changes)
2
2. Effects of earthworms on populations
of plant pests, parasites, and
pathogens
(increase or decrease in
populations and incidence of plant-
parasitic nematodes, phytopathogenic
fungi and bacteria, plant viruses?, shoot-
and root-feeding insects)
3. Production of plant growth
promoting/regulating substances
(hormones, vitamins, humic matter,
auxins, cytokinins, gibberellins,
ethylene, microbially induced and/or
excreted by earthworms.
Direct
(earthworm
activities that
influence root
growth/production in
a direct manner)
4. Root abbrasion and ingestion of
living plant parts by earthworms
(feeding and/or ingestion by earthworms
of living roots or plant shoots, and direct
damage to growing roots)
5. Interactions between earthworms
and seeds
(ingestion, digestion, burial,
dispersal, changes in germination rates
and potential)
likely to affect microorganisms in the litter layer and the roots growing through the organic matter/humus
(O/H) horizons and the soil surface-litter interface compared with the endogeic (soil-dwelling) geoph-
agous (soil-feeding) earthworm species, which tend to have a greater effect on microorganisms living
within the soil. Anecic, litter-burying species of earthworms, which create deep vertical burrows and
surface middens (small mounds of leaves blocking the entrance of vertically oriented burrows connected
to the soil surface) can have a major influence on microorganisms (fungi, bacteria, actinomycetes) and
micro-, meso-, and macroinvertebrates (protozoa, nematodes, mites, springtails, enchytraeids, milli-
pedes, isopods, other earthworms) in surface litter communities (Brown 1995; Anderson and Bohlen
1998; Maraun et al. 1999). However, their effects on the microbial communities living within the soil
are probably less than those of endogeic species because of their decreased soil-burrowing activities
as they tend to build more permanent burrow systems. Nevertheless, anecic earthworm species (and
large endogeic species) often have burrows that reach depths of more than 2 m, which can represent
important pathways of microbial dispersal and hot spots of microbial and root growth activity compared
with that in the surrounding soil matrix (Bhatnagar 1975; Ehlers et al. 1983).
 
 
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