Agriculture Reference
In-Depth Information
T3
700 mg (10 earthworms)
31% MICORRIZAE
T2
350 mg (5 earthworms)
30% MICORRIZAE
T1
0 mg (no earthworms)
6.6% MICORRIZAE
FIGURE 2.4
Stimulation of
Eugenia stipitata
(araz) growth and root mycorrhizal colonization 120 days
after inoculating tree nursery bags (filled with 2 parts soil and 1 part composted sawdust) with five (0.35 g
total wet weight) or ten (0.7 g) individuals of the pantropical geophagous endogeic earthworm species
P.
corethrurus
. (Ydrago 1994; Photograph P. Lavelle.)
Dispersal of mycorrhizal propagules (hyphae, infected root fragments, spores) has been reported
by various authors (McIlveen and Cole 1976; Rabatin and Stinner 1988; Ponge 1991; Reddell and
Spain 1991a; Gange 1993; Lee et al. 1996; Cavenden et al. 2003), and although some hyphae and
spores may be digested, many are still infective after passage through the earthworm gut (Reddell
and Spain 1991a; Gange 1993). Mycorrhizal dispersal and deposition of earthworm casts in the
rhizosphere may benefit root colonization by fungi, aid plant establishment in early successional
stages, and contribute to the heterogeneous nature of mycorrhizal distribution in soil communities
(Gange 1993). For example, the pantropical geophagous endogeic earthworm species
P. corethrurus
increased colonization of roots by arbuscular mycorrhizae in various tropical tree seedlings (Ydrogo
et al. 1994; Figure 2.4) and a pasture grass (Brown et al. 2000), also increasing plant biomass on
several occasions. The actinomycete
Frankia
and ectomycorrhizae were also shown to be dispersed
by
(Reddell and Spain 1991b; Reddell et al. unpublished), although the effects of
this on plant productivity are little known. Nevertheless, soil bioturbation and feeding in the
rhizosphere by earthworms may break up extramatrical hyphae and the hartig net, thereby reducing
root colonization by these root symbionts, hence providing potential benefits to the plants (Pattinson
et al. 1997; Brown et al. 2000; Tuffen et al. 2002).
Plant growth-promoting rhizobacteria (PGPR) such as
P. corethrurus
Enterobacter cloacae
,
Azotobacter
,
Azospirillum
spp. may also be dispersed and their
populations or activity increased in the drilosphere (Bhat et al. 1960; Kozlovskaya and Zdhanni-
khova 1961; Kozlovskaya and Zaguralskaya 1966; Bhatnagar 1975; Loquet et al. 1977; Hand and
Hayes 1983; Savalgi and Savalgi 1991; Pederson and Hendriksen 1993). The metabolites released
by these microorganisms may be particularly important to the potential plant responses (mechanism
3). Dispersal of these and other microorganisms such as biocontrol bacteria (e.g.,
,
Acinetobacter
,
Bacillus,
and
Pseudomonas
Pseudomonas
corrugata
) that colonize the rhizo-
sphere and prevent root diseases needs further investigation. The dispersal of various symbiotic
N
) and fungi (e.g.,
Gliocladium virens
,
Trichoderma harzianum
in clover; Doube et al.
1994a) also needs further research (Stephens et al. 1994e; Stephens and Davoren 1994; Singer et
al. 1999). These microorganisms all have an inability to spread actively and rapidly through the
-fixing rhizobacteria that nodulate legume roots (e.g.,
Rhizobium trifolii
2
