How Earthworms Affect Plant Growth: Burrowing into the Mechanisms - Earthworm Ecology - page 17

PLANT

OTHER ORGANIC MATTER
SOURCES

Nutrient
absorption
Rhizo-
deposition

ORGANIC
MATTER

Excretions,
secretions,
dead tissue

MICROBIAL
POPULATIONS
and ACTIVITY

H 2 O and air
circulation EARTHWORMS

CASTINGS

BURROWS

Better root
penetration

SOIL

Simplified conceptual model connecting the physical, chemical, and biological earthworm
effects on soils with their potential effects on plant growth and nutrition. (Modified from Cuendet and Bieri,
1999; Syers and Springett, 1983.)

FIGURE 2.2

1. D C P A B

ISPERSAL AND HANGES IN OPULATIONS AND CTIVITIES OF ENEFICIAL

M

ICROORGANISMS

Large populations of beneficial (plant growth promoting [PGP]) microorganisms such as saphrophytic
and mycorrhizal fungi, actinomycetes (e.g., ), bacteria, and microinvertebrates, such as pro-

tozoa and microbivore (fungivorous, bacteriophagous, predatory omnivorous and entomophathogenic)
nematodes inhabit the soil. Nevertheless, because of their limited ability to disperse within the soil
and the soil environmental and nutritional limitations to their activities, a large proportion of soil

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microorganisms are inactive at any given time, waiting for suitable conditions to promote higher levels

of activity (Lavelle 1997). Invertebrate activities, such as earthworm burrowing and casting, promote
soil mixing and bring microorganisms into contact with inaccessible soil resources, stimulating both
their populations and their activity. The earthworm gut also provides an ideal environment for enhanced
activity levels or multiplication of some microorganisms; others may be digested or their activity
levels reduced by passage through the earthworm gut (Brown et al. 2000). The complex resulting
effects of earthworms on microbial communities in soils (activity, populations, diversity) depend on
the reactions of microorganisms to passage through the earthworm gut and the ability of microorgan-

isms to utilize the drilosphere. Thus, earthworms may affect microbial populations (beneficial, fac-

ultatively pathogenic, and adverse species) directly, by feeding and digestive processes or indirectly
by burrowing and casting activities, which change root growth and development and the soil envi-

ronment, thereby making it more or less favorable to the development of microorganisms ( Figure

2.3 ) . Furthermore, as earthworms move through the soil matrix, they may disperse microorganisms,

both superficially (on the earthworm body) or via ingestion-egestion (in casts).
The ability of earthworms to disperse microorganisms or stimulate microbial activity and increase
microbial populations depends greatly on the earthwormÔs spatial range of activity, food requirements
and sources, and behavior. Epigeic, litter feeding, and dwelling species of earthworms are much more