Environmental Engineering Reference
In-Depth Information
ranges from 100 to 2,000 mg P kg
−1
soil repre-
senting approximately 350-7,000 kg P ha
−1
in
surface 25 cm of the soil, although only a small
portion of this P is immediately available for
crop uptake (Morel
2002
). However, for AM
fungi which are known for its P uptake (Hu et al.
2009
), soil P is one of the major deterrent soil
factors in agricultural systems that affects AM. It
has been well proved that the AM fungal benefi t
tends to decline as the concentration of P in the
plant increases (Valentine et al.
2001
). Higher
tissue P reduces the production of external
hyphae (Bruce et al.
1994
), hyphal branching
(Nagahashi and Douds
2000
) and sporulation
(De Miranda and Harris
1994
) of AM fungi. As
available P in the soil increases, AM association
may depress plant growth, as there is a carbon
cost associated with supporting the association
(Kahiluoto et al.
2000
). For example, cucumber
plants inoculated with AM fungi and raised on
full-strength nutrient solution had 19 % lower
biomass than uninoculated plants (Valentine
et al.
2001
). In contrast, mycorrhizal plants had
66 % higher biomass compared to non-mycor-
rhizal plants under reduced P concentration in
the nutrient solutions. However, the effect of P
fertilization on AM fungi may vary with P
sources. While readily soluble or available,
forms of P (inorganic) affect AM association to a
greater extent than less soluble forms of P (e.g.
rock phosphate) (Linderman and Davis
2004
).
Similar results have also been observed for N
fertilization (Gryndler et al.
1990
; Liu et al.
2000
). The effect of P fertilization often changes
with the response or balance of other nutrients
present. The AM fungal benefi t and mycorrhiza-
tion tend to be highest when low P is combined
with an ample supply of other nutrients (Grant
et al.
2005
). For instance, Guttay and Dandurand
(
1989
) observed an increased mycorrhization in
corn with N and K fertilization at low P levels
but a decrease at high P levels. This clearly sug-
gests the interactions among N, P and K fertil-
ization in corn. The application of NPK fertilizer
along with AM fungi has been shown to increase
plant growth in potato (Eliopoulos et al.
2007
),
onion (Gergon et al.
2008
) and cucumber
(Ahmed et al.
2009
). Using AM as a bioinocu-
lant, instead of phosphate, was found to have a
direct impact on sugarcane growth and yield
(Surendran and Vani
2013
). Like for AM coloni-
zation, some reliable evidence does indicate that
the use of fertilizers can reduce AM fungal spore
populations in the soil (Bhadalung et al.
2005
;
Emmanuel et al.
2010
).
In organic farming, the use of synthetic
fertilizers is avoided which enables the crops to
depend on AM fungi for soil nutrients (Galvez
et al.
2001
). In addition, organic fertilization
enhances AM fungal association and formation
of AM fungal propagules in the soil (Gryndler
et al.
2005
; Gaur and Adholeya
2005
), thereby
improving soil quality. Though organic fertil-
izers generally have a positive effect on crops
as evidenced by enhanced growth and accumu-
lation of nutrients (Silva et al.
2007
; Sharif
et al.
2012
), results of some studies are found to
be opposite (Martin et al.
2002
; Elorrieta et al.
2003
). A consortium of seven AM fungi iso-
lated from the soils of coffee (
Coffea arabica
)
plantations with different fertilizer inputs (low,
intermediate and high) was examined for their
growth-promoting ability in coffee both under
nursery and fi eld conditions. The results of this
study clearly showed that greater fertilizer
inputs negatively infl uenced the spore abun-
dance and plant growth, whereas intermediate
input increased the AM fungal abundance
(Trejo et al.
2011
).
4.2
Tillage
Tillage is an integral part of modern agriculture
that modifi es the physical, chemical and biolog-
ical properties of a soil. Consequently, tillage
practices may also affect AM fungi (Gálvez
et al.
2001
; Kabir
2005
; Neelam et al.
2010
).
The extent of extraradical AM fungal networks
can be several metres per cubic centimetre of
soil, providing the major nutrient-absorbing
interface between plants and soil (Jakobsen
et al.
1992
). The persistence of AM fungi in eco-
systems depends on the formation and survival
of propagules (e.g. spore, hyphae and colonized
roots). While spores are considered to be a resis-
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