Agriculture Reference
In-Depth Information
Beneficial plant-growth promoting (PGP), facultative, or obligatory
pathogenic rhizosphere microorganisms
Earthworm
feeding
Time
Soil/rooting
environment
Root growth
and development
Earthworm burrowing
and casting
FIGURE 2.3
Interactions among beneficial, facultative and obligatory plant-pathogenic rhizosphere micro-
organisms, earthworms, plant roots and the abiotic root environment, determining plant root growth and
development (note: this is a modified version of the classic Ñplant disease triangleÒ taught in plant pathology).
It has often been suggested that earthworms tend to promote changes in the microbial commu-
nity toward a bacterial-based trophic chain. Actually, phospholipid fatty acid (PLFA) methyl esters
analyses of earthworm-worked soils indicated that Gram-negative bacteria seem to be favored
compared with Gram-positive bacteria (Clapperton et al. 2001; Enami et al. 2001). Lumbricid
earthworms also increase bacterial-to-fungal ratios (Clapperton et al. 2001), although when a plant-
pathogenic fungus was inoculated into the soils, earthworms decreased this ratio, implying that
they may also increase the soil fungal biomass. Nevertheless, several species of fungi have been
shown to be ingested preferentially by the earthworm
(Moody et al. 1995;
Cooke 1983; Moody et al. 1995; Bonkowski et al. 2000), and Edwards and Fletcher (1988) reported
that fungi were a major food source for earthworms. This implies that earthworms (particularly the
litter-burying or fragmenting anecic and epigeic species) may impose some selection pressures on
fungal populations in both litter and soils. Bacterial-to-fungal ratios in soils are also often greater
in earthworm-worked soils because bioturbation tends to affect fungal populations negatively more
than those of bacteria (Hendrix et al. 1986).
The rhizosphere, a less-than-0.5-mm soil layer surrounding plant roots, is rich in microorgan-
isms, with species that are beneficial or adverse to root growth. Several earthworm species (espe-
cially some endogeics) seem to feed mainly in the rhizosphere (James and Seastedt 1986; Rovira
et al. 1987; Robertson et al. 1994; Hirth et al. 1998). Activity of lumbricid earthworms has been
reported in the rhizosphere of a temperate pasture (Carpenter 1985) and of wheat (Doube and
Brown 1998), and feeding in the rhizosphere was inferred from radio- (
Lumbricus terrestris
C) or stable isotope (
N,
14
15
C) analyses of the tissues of earthworms (
L. terrestris
and
P. corethrurus
) living in soils under
13
various plants (wheat, maize,
, and sugarcane) (Spain et al. 1990; Spain and
Le Feuvre 1997; Cortez and Bouch 1992; Brown 1999). There are also records of earthworms
feeding on living and dead root tissues (see mechanism 4), but the role of root tissues and their
derivatives (rhizodeposition) in earthworm diets remains little understood (Brown et al. 2000).
Earthworm feeding or movement in or around the rhizosphere can have important consequences
for associated microbial and faunal communities (activity, populations, diversity) and thus, indi-
rectly, on plant productivity (Figure 2.3).
Brachiaria decumbens
