Agriculture Reference
In-Depth Information
soil temperature, pH, and humidity, nitrification occurs at a very fast rate. Sahrawat (2008) and De
Boer and Kowalchuk (2001) reported that the
Nitrosomonas
and
Nitrobacter
bacterial responsible
for the nitrification process is pH sensitive, and thus, nitrification proceeds slowly in acid soils.
Sahrawat (1982) studied nitrification in 10 soils (eight mineral and two Histosols) having a range
in texture, pH (3.4-8.6), organic C (12.2-227.0 g kg
−1
), and total N (0.9-1.2 g kg
−1
). The amounts
of NO
3
-N produced at 30°C after 4 weeks of incubation of the soils varied from 0 to 123 mg kg
−1
soil. Soils with pH < 5.0 did not nitrify at all; the organic soil with pH 5.6 produced only 5 mg kg
−1
NO
3
-N soil during the period. Soils having pH > 6.0 nitrified at a rapid rate and released NO
3
-N
ranging from 98 to 123 mg kg
−1
soil.
Several studies have shown that oxidation of
NH
4
+
to
NO
3
−
(nitrification) is one of the major
cause of soil acidity in many ecosystems (Sahrawat, 2008; Rodriguez et al., 2008). The nitrification
reaction releases protons (H
+
), which result in acidification of the soil when ammonical and most
organic N sources are converted into nitrate. According to the microbial-mediated two-step chemi-
cal reaction (Ross et al., 2011), conversion of one mole of
NH
4
+
into
NO
3
−
results in the production of
two moles of H
+
into the soil environment to cause soil acidity.
In addition, nitrification is an oxidation process and aeration of soil increases nitrification.
Ploughing and cultivation are recognized means of promoting nitrification. Nitrification results in
the release of H
+
ions, leading to soil acidification. Furthermore, the enzymatic oxidation of nitrifi-
cation also releases energy. The utilization of
NH
4
+
and
NO
3
−
by plants and microorganisms consti-
tutes assimilation and immobilization, respectively.
The concentration of N
2
O has been rising at an increasing rate, particularly since about 1960.
Cu r rently, N
2
O concentrations in the atmosphere are increasing at about 0.3% per year (Seiler,
1986). The atmosphere contains about 1500 Tg (Terra gram = 10
12
g) of N
2
O. Perhaps, 90% of the
emissions are derived from soils through biologically mediated reactions of nitrification and deni-
trification (Byrnes, 1990). Emission of N
2
O may be perceived as a leakage of intermediate products
in each of these processes. While soils are a major source of N
2
O, they also serve as a sink (Freney
et al., 1978; Fageria and Gheyi, 1999).
The fixation of
NH
4
+
is an important component of the N cycle in soil-plant systems. Three
specific processes are recognized as responsible for the fixation and retention of N applied to soil,
namely, fixation by clay minerals, NH
3
fixation by soil organic matter, and biological immobiliza-
tion of NH
4
by heterotrophic microorganisms (Nommik and Vahtras, 1982). Adsorption of
NH
4
+
in a nonexchangeable form in the interlayer region of expanding 2:1—layer aluminosilicate clay
minerals (fixation) may reduce the fertilizer—use efficiency of this nutrient when added to soils in
which such minerals predominate (Stehouwerand and Johnson, 1991). This type of fixation was first
reported early in this century and many studies have been conducted to investigate this phenomenon
with respect to fertilizer use efficiency (Nommik and Vahtras, 1982). A greater fixation of NH
4
+
has
been reported with anhydrous NH
3
(AA) than with other forms of
NH
4
+
-releasing fertilizers (Young
and Cattani, 1962). This may be due to the enhanced reaction of NH
3
with more acidic water in 2:1
clay interlayer (Nommik and Vahtras, 1982). Many earlier studies reported that the increase in pH
increased fixation of
NH
4
+
(Nommik, 1957). This effect is generally attributed to decreased competi-
tion with H
3
O
+
as pH increased in a lower pH range (2.5-5.5), and to decreasing the charge of inter-
layer hydroxy-Al polycations in a higher pH range (5.5-8.0). An increasing solution concentration
of
NH
4
+
has been shown to increase the amount of
NH
4
+
fixed, while the percentage fixation (amount
fixed/amount added) was decreased (Black and Waring, 1972). The removal of organic matter from
mineral surfaces has been found to increase
NH
4
+
fixation (Hinman, 1966).
Fixed
NH
4
+
is involved in the N dynamics of soil, and may be an important component of the N
fertility status of some agricultural soils. Between 18% and 23% of added
15
NH
4
was fixed (spe-
cifically adsorbed) after 15 days of incubation in soils containing relatively high amounts of ver-
miculitic clay (Drury et al., 1989). Ammonium
15
N has been observed to be fixed and released
in proportion to added
15
NH
4
and vermiculite content (Keerthisinghe et al., 1984). Drury and
Beauchamp (1991) reported that immobilization of 5.7% of the added
15
NH
4
occurred in the high
