Agriculture Reference
In-Depth Information
anaerobic conditions by methylation into volatile compounds. The reactions
include (Reamer and Zoller, 1980):
methylation
−−−−−→
CH
3
SeO
3
H
reduction
HSeO
3
−
CH
2
SeO
2
−
−−−−→
methylation
−−−−−→
reduction
−−−−→
(
CH
3
)
2
Se
↑
CH
3
SeO
2
−
(
CH
3
)
2
SeO
2
↑
or CH
3
SeOOCH
3
↑
reduction
−−−−→
CH
3
SeO
2
−
CH
3
SeH or CH
3
SeOH
−−−→
CH
3
SeSeCH
3
↑
The different steps are mediated by a consortium of microbes with tolerances to
the various form of Se, resulting in removal of toxic Se from the soil though
enhancing atmospheric Se transport.
Iodine
Iodine is essential in the mammalian diet to produce the thyroid hormone thy-
roxine; deficiency in humans causes goitre. Collectively, deficiencies of iodine,
iron, zinc and vitamin A in humans are thought to be at least as widespread and
debilitating as calorie deficiencies (Welch and Graham, 1999). The main source
of iodine in soils is oceanic salts rather than parent rock, and so deficiency is
most widespread in areas remote from the sea (Fuge, 1996). In principle defi-
ciency is easily corrected with dairy supplements. However in practice this is not
always feasible. Addition of iodate to irrigation water has successfully corrected
widespread iodine deficiency in parts of China where the usual methods of sup-
plementation had failed (Cao
et al
., 1994; Jiang
et al
., 1997). However there is
not much information on the behaviour of iodine in soil and water systems.
Iodine is present in the environment predominantly in the oxidation states
−
1
(I
−
, iodide) and
5 (IO
3
−
, iodate). Reduction of IO
3
−
to I
−
occurs at pe
0
+
=
13
.
3atpH5andpe
0
11
.
3atpH7.HenceI
−
is expected to predominate
in the soil solution except in oxic alkaline soils (Whitehead, 1984). However
Yuita (1992) found predominantly IO
3
−
in acid Japanese soils contaminated with
iodine: the concentrations in solution were some 20 times those of I
−
and I
2
.
On flooding the soils, the total concentration of I in solution increased 10- to
50-fold, predominantly as I
−
. The concentrations of sorbed I were not measured,
but both IO
3
−
and I
−
are expected to be bound to organic matter and oxides and
hence their concentrations in solution are expected to increase with reductive
dissolution reactions. Further, for a given concentration in solution, I
−
is more
rapidly absorbed by plants than IO
3
−
(Mackowiak and Grossl, 1999). Hence
flooding is expected to increase accumulation in plants both through increased
solubility and increased absorption.
=
