Agriculture Reference
In-Depth Information
Substituting from Table 3.3 for the equilibrium constant for dissociation of
H
2
CO
3
,whichisfast,
=
k
f
1
+
k
f
2
[CO
2
]
−
k
b
1
+
k
b
2
K
H
2
CO
3
[H
2
CO
3
]
d[CO
2
]
d
t
−
(
3
.
21
)
or
d[CO
2
]
d
t
k
H
2
CO
3
K
H
2
CO
3
[H
+
][HCO
3
−
]
−
=
k
CO
2
[CO
2
]
−
(
3
.
22
)
where
k
CO
2
=
k
f1
+
k
f2
and
k
H
2
CO
3
=
k
b1
+
k
b2
K
H
2
CO
3
. Equation (3.22) corres-
ponds to the simplified scheme
k
CO
2
−−−→
fast
−−−→
H
+
+
HCO
3
−
CO
2
+
H
2
O
H
2
CO
3
(
3
.
23
)
←−−−
k
H
2
CO
3
←−−−
That is, the hydration reaction is first order with respect to dissolved CO
2
.Therate
constant
k
CO
2
=
0
.
025-0
.
04 s
−
1
(
25
◦
C
)
and activation energy
≈
63 kJ mol
−
1
.For
the dehydration reaction,
k
H
2
CO
3
=
10-20s
−
1
(
20-25
◦
C
)
and activation energy
≈
67 kJ mol
−
1
.
3.3.1 FLOODWATER CO
2
DYNAMICS
The pH of the water on the surface of a submerged soil often depends on the
activity of photosynthetic organisms. Photosynthesis by aquatic plants and algae
removes dissolved CO
2
during the day, but at night the net respiratory activity of
the organisms returns CO
2
to the water and the concentration of dissolved CO
2
and acidity increase:
photosynthesis
−−−−−−−→
CO
2
+
+
H
2
O
CH
2
O
O
2
(
3
.
24
)
←−−−−−−−
respiration
where CH
2
O is organic matter produced in photosynthesis or consumed in respi-
ration. As a result the pH may rise as high as 10 during the day but fall by two
or three pH units at night. Figure 3.2 shows measured diurnal changes in pH and
carbonate species in the floodwater of a ricefield. The relations between pH, alka-
linity and carbonate equilibria are described by Equation (3.2). Equation (3.24)
shows that photosynthesis and respiration do not affect the alkalinity of the water
per se
. The pH increases or decreases with the change in
C
T
at constant alkalin-
ity. The change in pH depends on the alkalinity as it affects the initial pH and the
consequent acid-base system operating. At pHs below p
K
1
(
=
6
.
3
),
CO
2
(aq) is
the dominant species and there is little change in pH with
C
T
. Between p
K
1
and
p
K
2
(
=
10
.
3
)
HCO
3
is the dominant species and roughly 1mol of H
+
is consumed
per C fixed in photosynthesis
(
HCO
3
−
+
H
+
→
CH
2
O
+
O
2
)
, with a correspond-
ingly greater pH change. At pHs above p
K
2
,
CO
3
2
−
is the dominant species and
roughly 2mol of H
+
are consumed per C fixed
(
CO
3
2
−
+
2H
+
→
CH
2
O
+
O
2
)
,
