Agriculture Reference
In-Depth Information
15
10
with CO
2
hydration
5
0
without
−
5
−
10
0.01
0.1
1
10
100
1000
CO
2
pressure (Pa)
Figure 3.6
Flux of CO
2
as a function of CO
2
pressure with and without carbonate
equilibria
Figure 3.6 shows how the flux of CO
2
across the interface varies with CO
2
pressure in the bulk solution, with and without equilibration between CO
2
and
carbonate species in the boundary layer. A positive flux indicates dissolution and
a negative flux volatilization. The figure shows that the effect of the carbonate
equilibria is very marked at small CO
2
pressures, but insignificant at large pres-
sures where transport across the boundary layer is primarily as H
2
CO
3
∗
.Atsmall
CO
2
pressures the rate of dissolution is enhanced many fold by the carbonate
equilibria, the effect increasing as the CO
2
pressure decreases and the pH of the
bulk solution correspondingly increases.
An important practical problem in ricefields is the loss of N fertilizer through
volatilization of NH
3
from the floodwater. Loss of NH
3
is sensitive to the pH
of the floodwater, and hence is intimately linked to the dynamics of dissolved
CO
2
(Bouldin and Alimago, 1976). To quantify this it is necessary to consider
the simultaneous transfers of CO
2
and NH
3
across the air-water interface and
their coupling through acid-base reactions. There is an equation of type (3.33)
forthefluxofNH
3
across the still air layer and, as for the dissolved CO
2
and
carbonate species, the flux across the still water layer is
F
GN
=
F
LN
=
F
LNH
4
+
+
F
LNH
3
+
F
LNH
4
OH
(
3
.
39
)
The acid-base pairs involved are NH
4
+
-NH
3
and NH
4
+
-NH
4
OH, in addition
to those listed above, and we have
F
GC
−
F
GN
=
F
LH
2
CO
3
−
F
LCO
3
2
−
−
F
LNH
3
−
F
LNH
4
OH
−
F
LOH
(
3
.
40
)
Equation (3.40) is inherent in the mass and charge balances. These equations can
be solved as before to calculate the simultaneous fluxes of CO
2
and NH
3
across
the air-water interface.
