Environmental Engineering Reference
In-Depth Information
the nutrients are removed from the soil solution
by the plant roots (Fig.
4
). For poorly mobile ions
such as phosphate, a sharp and narrow depletion
zone develops very close to the root. Hyphae of
AM fungi can readily spread beyond this deple-
tion zone and take up additional phosphate from
the soil (Li et al.
1991
) (Fig.
4
). The uptake of
other nutrients like N, K and micronutrients is
also improved by AM fungi because many of
these elements are also limited due to various
reasons in the soil.
Two important factors that contribute to the
effective uptake of nutrients by AM fungi from
the soil are (i) the narrow diameter of the fungal
hyphae and (ii) its longer lifespan relative to root
and root hairs. As the diffusion gradient for a
nutrient is inversely related to the radius of the
absorbing unit, the soil solution should be less
depleted at the surface of a narrow absorbing unit
such as hyphae. Further, narrow hyphae can also
grow into small soil pores that are not accessible
to roots and root hairs (O'Keefe and Sylvia
1991
). Therefore, crop species with well-
developed root systems with fi ne roots or abun-
dant root hairs like wheat, barley and oats (
Avena
sativa
) remains little affected by AM coloniza-
tion (Ryan and Graham
2002
).
3
Effects of AM Fungi
The major effects of AM association on host
plants include enhanced uptake of low-mobile
ions, nutrient cycling, rooting and plant estab-
lishment, plant tolerance to various biotic and
abiotic stresses, improved soil quality and
structure and enhanced plant community diver-
sity. In agricultural ecosystems, AM fungi play
a vital role in maintaining sustainability
(Sanders
2004
), by enhancing crop growth
(Meir et al.
2010
) and productivity (Lekberg
and Koide
2005
), soil constituents and fertility
(Piotrowski et al.
2004
; Li et al.
2007
) and
pathogen resistance (Sikes et al.
2009
).
3.1
Improved Nutrient Uptake
and Nutrient Cycling
Arbuscular mycorrhizal fungi improve plant
uptake of nutrients by increasing the plant sur-
face area of absorption. The narrow diameter of
the absorbing hyphae allows more nutrients to be
taken up from the soil solution. Generally, nutri-
ent depletion zones develop around the root when
Fig. 4
Arbuscular mycorrhizal fungal hyphae and nutrient depletion zones around root
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