Environmental Engineering Reference
In-Depth Information
hyphal growth and establishment (Douds et al.
2000
), consequently aiding the transfer of min-
eral N (Hamel
2004
) and amino acids
(Govindarajulu et al.
2005
) from the organic
manures to the host plant. Minimal C:N ratio in
the organic manures also has a positive infl uence
on AM fungi (Groaker and Sreenivasa
1994
).
Likewise, the application of organic manures to
soils with different nutrient levels affects the AM
fungal colonization and abundance variedly.
Both soil factors and N:P ratio of host roots can
infl uence AM fungal colonization, but the mech-
anism remains unresolved (Liu et al.
2000
;
Johnson et al.
2003
). Available phosphorus and
mycorrhization in coconut (
Cocos nucifera
)
were found to be higher in organic manure-
amended soils than in inorganic fertilizer-applied
soils, even though the later contain higher
nutrient contents (Karunasinghe et al.
2009
).
The application of various combinations of
organic manures (farmyard + poultry + humic
acid) along with AM fungal inoculation showed
enhanced plant growth and nutrient uptake and
spore density in eroded soils (Sharif et al.
2012
).
This suggests AM fungal ability to reduce the
effect of soil erosion and shield the soil fertility
(Valarini et al.
2009
). Numerous studies have
also revealed the benefi cial effects of organic
manure application on AM fungi (e.g. Limonard
and Ruissen
1989
; Lee and Pankhurst
1992
;
Hole et al.
2005
). Dai et al. (
2011
) conducted
an experiment with various levels of organic
amendments on chilli and showed an increased
mycorrhizal colonization and a higher plant tis-
sue nutrient in response to organic amendments.
The application of organic manures not only
stimulates the AM fungal colonization of roots
but also improves in spore populations in the soil.
Organic manure amendment along with AM fun-
gal inoculation has been shown to enhance plant
growth and spore numbers of
C. etunicatum
and
F. mosseae
in soils than those fertilized with con-
ventional fertilizers and inoculated with AM
fungi (Douds et al.
2000
). During early stages of
plant growth, the spore numbers in organic
manure-amended soil tend to decline and then
increase subsequently due to alterations in nutri-
ent content arising from decomposing manures
(Harinikumar and Bagyaraj
1989
; Muthukumar
and Udaiyan
2002
; Gryndler et al.
2009
; Ijdo
et al.
2010
).
4.4
Biocides
Biocides, the chemical agents used to control
pests and pathogens, are an inherent component
of conventional agriculture. In plant production
systems involving horticultural crops such as veg-
etables, most cultivators are unwilling to risk low
production through reduced fertilizer or biocide
inputs. Information on the effect of agricultural
chemicals on AM fungi is largely empirical and
poorly understood. Thus, biocide application may
have inadvertent or unrecognized effects on AM
fungi. Biocides used with the intention on pro-
moting plant health may either impair or elimi-
nate AM fungal activity causing damage to plant
health. There are two benefi ciary effects of bio-
cides like fungicides on AM fungi: (a) modifi ca-
tion of host plant physiology by enhancing root
exudates that indirectly stimulate root coloniza-
tion and (b) reduction of AM fungal antagonistic
community (Tataranni et al.
2012
). Most studies
examining the effect of biocides on AM fungi are
often conducted under greenhouses or in plant
growth chambers, involving sterile media or
media which have very little similarity to fi eld
conditions (Udaiyan et al.
1995
). Soil fumigants
used to reduce the abundance of pathogen cause
stunting in a range of crops including onion, pep-
per (Hass et al.
1987
), soybean (Ross and Harper
1970
) and corn (Jawson et al.
1993
). This reduc-
tion in plant productivity has often been attributed
to decreased AM formation, which results in poor
nutrient uptake. However, results of fi eld studies
do suggest that the elimination of co-occurring
soil microorganisms might also substantially con-
tribute to this effect (Hetrick et al.
1988
).
Arbuscular mycorrhizal fungal responses to
biocides are varied and may be infl uenced by the
host plant, specifi c chemical compounds, method
of application, mode of action, growth stage of
AM fungi and biotic and abiotic factors
(Giovannetti et al.
2006
). Fungicides applied as
soil drenches generally have a detrimental effect
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