Agriculture Reference
In-Depth Information
in Figure 6.18, which shows the sensitivity of the pH change at the root surface
to the flux of H
+
across the root and the important soil variables.
6.5 CONSEQUENCES OF ROOT-INDUCED CHANGES
Effects of the root-induced changes on the general microbiology of submerged
soils are discussed in Chapter 5 and effects on methane production and consump-
tion are discussed in Chapter 8. I here discuss specific effects on plant nutrients.
6.5.1 NITRIFICATION-DENITRIFICATION IN THE RHIZOSPHERE
As we have seen in Section 6.4, wetland rice is particularly efficient at absorbing
NO
3
−
. Kirk and Kronzucker (2000) developed a model to calculate the extent
to which rice growing in submerged soil can capture NO
3
−
formed in the rhi-
zosphere before it diffuses away and is denitrified in the soil bulk. The model
allows for the following processes.
(1) Transport of O
2
away from a root and its consumption in microbial pro-
cesses—in addition to nitrification—and oxidation of mobile reductants such
as Fe
2
+
. Microbial O
2
consumption is described with Michaelis-Menten
kinetics and Fe
2
+
oxidation with first-order kinetics with respect to both
[O
2
]and[Fe
2
+
].
(2) Transport of NH
4
+
towards the root and its consumption in nitrification and
uptake at the root surface. Nitrification is described with dual Michaelis-
Menten kinetics allowing for [O
2
]and[NH
4
+
].
(3) Transport of NO
3
−
formed from NH
4
+
towards the root and its consumption
in denitrification and uptake by the root. Denitrification is described with
Michaelis-Menten kinetics with an inhibition function related to [O
2
].
Uptake of NH
4
+
and NO
3
−
into the root are described by Michaelis-Menten
relations with the parameter values discussed in Section 6.3.
Figure 6.19 shows the concentration profiles of O
2
,
Fe
2
+
,
NH
4
+
and NO
3
−
near a root calculated with this model for realistic flooded soil conditions and
realistic rates of O
2
release (last section); Figure 6.20 shows the corresponding
fluxes of O
2
out of the root and NH
4
+
and NO
3
−
in. The amount of N denitrified
in 10 days in the calculations corresponds to about 10% of the NH
4
+
initially in
the volume of soil influenced by the root. This is of the order of maximum rates
of denitrification reported in the literature for rice in flooded soil, indicating that
the model parameter values are indeed realistic.
The calculations indicate that quite large amounts of NO
3
−
may be absorbed
by rice in flooded soils, perhaps as much as a third of the total N absorbed
if soil conditions and water management prevent very thorough soil reduction.
This may explain, for example, the benefit of the elaborate water management
