Agriculture Reference
In-Depth Information
Table 7.1 Acid-base changes in nitrogen transformations in ricefields
[H + ]
[N]
Process
1. Hydrolysis of urea fertilizer
CO ( NH 2 ) 2 + 2H + + H 2 O 2NH 4 + + CO 2
1
2. NH 3 volatilization
NH 4 + NH 3 + H +
+ 1
3. Nitrification
NH 4 + +
NO 3 +
2H +
2CO 2
+
H 2 O
2CH 2 O
+
+
2
4. Denitrification
4NO 3 + 5CH 2 O + 4H + 2N 2 + 5CO 2 + 7H 2 O
1
5. Removal of NH 4 + in crop a
426CO 2
+ 12NH 4 + + H 2 PO 4 + 408H 2 O
C 426 H 855 O 426 N 12 P
11H +
+
414O 2
+
+
0 . 92
a Based on data of Dobermann and Fairhurst (2000) for mean mineral content of grain (for
clarity, K + , Ca 2 + , Mg 2 + , SO 4 2 , Cl , etc. omitted).
Nitrification of the NH 4 + does not cause net acidification, whether the NO 3
is absorbed by the crop or denitrified. Two mol of H + are formed per mol of
NH 4 + nitrified, resulting in a net addition of one H + per urea-N hydrolysed. If
the NO 3 is absorbed by the crop, 1mol of OH is exported from the roots per
mol of NO 3 absorbed, or if it is denitrified, 1mol H + is formed per mol of N
denitrified (Table 7.1, Process 4).
Note that the anaerobic processes causing pH changes following submer-
gence—reduction of ferric iron tending to increase the pH of acid soils and
accumulation of CO 2 tending to decrease the pH of alkaline soils—are reversed
upon drainage and reoxidation of the soil. Thus, unless there has been substantial
movement of acid or base out of the soil during submergence, as generally there
will not have been, the pH changes are reversed. Permanent changes of pH only
occur if the concentration of acid or base in the water entering the ricefield differs
from that leaving it.
Ferrolysis
An exception to these general rules is the formation of so-called ferrolysed
soils (Brinkman, 1970; van Breemen and Buurman, 1998). These occur under
particular hydrological and geological conditions in which there is prolonged
seasonal waterlogging of the soil and subsequent drainage as the regional water
table falls. Dissolved Fe 2 + formed in soil reduction displaces cations from the
soil exchange complex, and these are subsequently removed in leaching or runoff.
When the soil dries, adsorbed Fe 2 + is oxidized producing acidity which reacts
with the soil. The resulting H + -saturated clay is unstable and partly decomposes
to give Al-saturated clay and silica. Repeated cycles of this process can lead to
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