Environmental Engineering Reference
In-Depth Information
on AM fungi (Udaiyan et al.
1999
). In a recent
study, Rotor and Delima (
2010
) assessed the
infl uence of AM fungi with the addition of N fertil-
izer and biocides on corn growth and productivity.
The results of this study clearly suggested that
microbial inoculants could act as a substitute for
biocide application. But the effect of fungicides
on AM association can vary with host-fungal
combinations. For example, the dicarboximide
fungicide captan is known to stimulate mycorrhi-
zation of beans by
Glomus
spp. (De Bertoldi
et al.
1977
), had no effect on undetermined spe-
cies colonizing onion (El-Giahami et al.
1976
)
and reduced colonization by
F. mosseae
in corn
(Sutton and Sheppard
1976
). Fungicides can also
adversely affect different stages of AM fungal
development and function (Trappe et al.
1984
).
The infl uence of three commonly used fungi-
cides, i.e. benomyl, pentachloronitrobenzene,
and captan, tested on mixed culture of AM fungi
indicated that these fungicides could alter the
species composition of AM fungal community
(Schreiner and Bethlenfalvay
1996
). Nevertheless,
the biological response of AM to these fungicides
depends not only on the fungus-fungicide rela-
tionship but also on the prevailing environmental
conditions (Schreiner and Bethlenfalvay
1997
).
Systemic fungicides like carbendazim can com-
pletely inhibit P uptake by AM fungal hyphae
even when applied at recommended fi eld rates
(Kling and Jakobsen
1997
). In addition, carben-
dazim could disrupt hyphal P uptake at concen-
trations as low as 10 % of the recommended fi eld
dosage (Schweiger and Jakobsen
1998
).
However, under fi eld conditions, carbendazim or
a mixture of propiconazole and fenpropimorph
applied at recommended rate did not affect AM
colonization (Schweiger et al.
2001
). In contrast
to carbendazim, the benzimidazole fungicide
benomyl has to be applied at a much higher rate
than recommended levels to affect AM fungal
colonization of roots (Gange et al.
1993
).
Like certain fungicides, higher dosage of
pesticides like malathion and mancozeb reduces
plant growth parameters and also affects the
extent of mycorrhizal colonization (Saleh
Al-Garni
2006
). Unlike other pesticides, the
herbicide atrazine at lower concentration
decreases mycorrhizal colonization, whereas at
higher concentration, it stimulates colonization
(Huang et al.
2006
,
2007
). However, it has been
speculated that the application of higher con-
centrations of atrazine tends to induce tolerance
in AM fungi, although the real mechanism
behind the varied effect is yet to be elucidated
(Huang et al.
2007
).
4.5
Crop Rotation
In general, microbes in the soil affect the suc-
ceeding crop. From the biological view point,
crop rotation is essential for proliferation of AM
fungi (Douds et al.
2005
). The AM fungi which
proliferate with a host plant are not necessarily
those best at promoting the growth of other crops
in the rotation (Feldmann et al.
1999
). The prolif-
eration of such AM fungi has been attributed as a
cause to yield decline in continuous monoculture
(Schenck et al.
1989
). This had been noted in
soybean and sorghum grown as continuous crop-
ping and also by crop rotation, which refl ected
less mycorrhizal colonization in the former
method than the later (Ellis et al.
1992
). In addi-
tion, the diversity of the AM fungal community is
linked to the diversity and productivity of the
plant community (van der Heijden et al.
1998
;
Bever et al.
2001
). A more relaxed altitude
towards weed management may increase both the
diversity and effectiveness of the AM fungal
community when the crops are non-mycorrhizal
(Miller and Jackson
1998
; Feldmann and Boyle
1999
; Jordan et al.
2000
). This is important under
circumstances where the cultivation of non-host
crop such as
Brassica
spp. is known to reduce
AM fungal inoculum in the soil (Blaszkowski
1995
).
Pre-cropping enhances mycorrhizal inoculum
potential (Dodd et al.
1990a
,
b
; Karasawa et al.
2001
,
2002
). The enhancement of mycorrhiza
inoculum potential by a given pre-crop may
improve the mycorrhizal activity of a subsequent
crop in the rotation (Barea et al.
1993
). This is
because the fungi develop and sporulate mostly
in the roots of those plant species, which are most
susceptible to mycorrhizal colonization. Susceptible
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