Agriculture Reference
In-Depth Information
(6) Because very large concentrations of dissolved CO
2
develop in submerged
soil, in spite of root respiration the CO
2
pressure outside the root may be
greater than that inside it, resulting in a flow of CO
2
from the soil to the atmo-
sphere through the aerenchyma. Net removal of CO
2
by the root decreases
the concentration of the acid H
2
CO
3
near the root, and this may offset the
acidity produced in oxidation and excess cation uptake.
The net effects of these processes will depend on their rates versus the rates at
which the resulting changes are buffered by processes in the soil. In the following
sections I give available information for different soil conditions.
6.4.1 OXYGENATION OF THE RHIZOSPHERE
The extent to which wetland roots oxygenate their rhizospheres is a matter of
contention. There is little doubt that some O
2
is released: reddish-brown ferric
oxide deposits are frequently observed on the surfaces of wetland roots. But the
magnitude of release is debated and measured rates of release vary by more than
two orders of magnitude (Bedford
et al
., 1991; Sorrell and Armstrong, 1991;
Kirk and Le van Du, 1997). The flux of O
2
across a particular portion of the
root depends not only on the rate of O
2
transport through the root—which is
itself complicated by the effects of root type, age and condition—but also on the
strength of the sink presented by the external medium. In soil, the strength of
the sink depends on the rate of O
2
diffusion into the soil, its rate of consumption
by microbes and reaction with mobile reductants such as Fe
2
+
,andtherateof
Fe
2
+
diffusion towards the oxidation zone. There are also differences along the
root length. As a root grows through a portion of soil, a zone of Fe
2
+
depletion
arises where oxidation is intense in the region of the root tip, but is rapidly
filledinwhentheO
2
supply decreases as the root grows passed. Re-reduction
of Fe(III) is slow compared with oxidation of Fe(II). Hence the root tips are
generally white and free of ferric oxide deposits, whereas the older parts are
coloured orange-brown.
The calculations in Section 6.2 indicate that the root system as a whole can sus-
tain considerable rates of O
2
loss to the rhizosphere without compromising their
internal O
2
requirements. The standard O
2
flux in the calculations in Section 6.2
was 0
.
5nmoldm
−
2
(root) s
−
1
for the parts releasing O
2
. For rice roots grown in
soil, Begg
et al
. (1994) obtained values of 0
.
1-1
.
2nmoldm
−
2
(root) s
−
1
from
rates of Fe
2
+
oxidation and Fe(III) accumulation near planar layers of rice roots
in anaerobic soil, and Kirk and Bajita (1995) obtained 0
.
1-0
.
2nmoldm
−
2
(root)
s
−
1
with the same experimental system but a soil with a smaller ferrous iron
content. These values probably underestimate the total O
2
release because they
did not allow for O
2
consumed by soil microbes. Revsbech
et al
. (1999) obtained
values of 1-3 nmol dm
−
2
(root) s
−
1
from measurements of O
2
gradients made
with a microelectrode near rice roots in the soil used by Kirk and Bajita (1995).
These values are in the middle of the range described above.
