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Figure 4.1. Schematic diagram of rhizospheric mobilization, root uptake and frond translocation of arsenic
in
P. vittata
. DOC, dissolved organic carbon and AMF, arbuscular mycorrhizal fungi.
4.2.2.1
Arsenic mobilization via root exudates
Both root exudates and bacteria associated with
P. vittata
have been shown to help arsenic sol-
ubilization in the rhizosphere. In a greenhouse study,
P. vittata
excreted 2 times more dissolved
organic carbon (DOC) than a control non-hyperaccumulating fern
Nephrolepis exaltata
(Tu
et al
.,
2004b). As a result, the organic acids from root exudates of
P. vittata
induced 3-18 times higher
mobilization of arsenic from insoluble arsenic minerals (AlAsO
4
and FeAsO
4
) and an arsenic-
contaminated soil as compared to
N. exaltata
. Besides root exudates, arsenic-resistant bacteria
inhabiting
P. vittata
rhizosphere (
Pseudomonas
sp.,
Comamonas
sp. and
Stenotrophomonas
sp.)
have been shown to exhibit a remarkable ability to increase arsenic concentration in the uptake
solution from
<
5
gL
−
1
to 5.04-7.37 mg L
−
1
by solubilizing insoluble FeAsO
4
and AlAsO
4
(Ghosh
et al
., 2011). The production of pyochelin-type siderophores by arsenic-resistant bacteria
has been suggested to play a role in arsenic solubilization.
To further investigate the rhizosphere characteristics of
P. vittata
relevant for its use in phytoex-
traction and the effects of root uptake on arsenic redistribution in soils, a sequential extraction
procedure has been developed to fractionate arsenic into five operationally-defined fractions with
decreasing availability (Fitz and Wenzel, 2002). It includes non-specifically bound, specifically
bound, bound to amorphous hydrous Fe/Al oxides, bound to crystalline hydrous Fe/Al oxides,
and residual fractions. In comparison with non-hyperaccumulator
N. exaltata
,
P. vittata
was more
efficient to access arsenic from all five fractions, leading to greater removal of arsenic (39-64%
vs
. 5-39%) from rhizosphere soils than
N. exaltata
(Gonzaga
et al
., 2006). This observation seems
related to 9% higher DOC and 0.5 unit higher pH in
P. vittata
rhizosphere, which tend to increase
arsenic bioavailability by facilitating arsenic desorption from solid phase via anion competition.
In addition, the majority of arsenic removed by
P. vittata
was from the major arsenic sink in soils
(i.e., bound to Fe/Al hydrous oxides), accounting for 68% arsenic decrease in the rhizosphere.
Fitz
et al
. (2003) reported that the difference in non-specifically bound arsenic (readily labile)
between bulk and rhizosphere soils accounted for only 8.9% of total arsenic accumulated in
P. vittata
, again suggesting arsenic uptake was mainly from less available pools. Compared with
µ
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