Agriculture Reference
In-Depth Information
Important Transformations of N: Mineralization and Immobilization
4.3
Depending on the soil and environmental conditions, the quantity of N in the
vine root zone (ca. 1 m deep) ranges from 1 to 10 t/ha, most of which occurs in
the top 15-25 cm. Organic N (in proteins, nucleic acids, and complex ligno-
protein compounds) is transformed to NH
4
ions during the decomposition of
organic matter by heterotrophic microorganisms—the process of
ammonification.
A simplified description of the process is
Organic N
NH
4
OH
(4.1)
which indicates an alkalizing reaction. This reaction may take place under aero-
bic or anaerobic conditions, but it is slower in the latter case because less energy
is available for microbial growth.
In well-aerated soils, ammonification is usually followed by the oxidation of
NH
4
to NO
3
by autotrophic bacteria. The principal nitrifying organisms are
of the genera
Nitrosomonas
and
Nitrobacter
. These organisms derive energy for
growth solely from the oxidation of NH
4
, which is coupled to the reduction of
CO
2
for the synthesis of complex C compounds. The oxidation occurs in two
steps:
3
NH
4
O
2
NO
2
2H
H
2
O
Energy
(4.2)
2
is carried out by
Nitrosomonas
species and
1
NO
2
O
2
NO
3
Energy
(4.3)
2
is carried out by
Nitrobacter
species. The overall process, called
nitrification,
is rep-
resented by the reaction
NH
4
2O
2
NO
3
H
2
O
2H
Energy (4.4)
NH
4
and NO
3
ions comprise the pool of
mineral N
on which plants feed.
The incorporation of mineral N into complex N compounds in living organisms
is called
immobilization
(section 2.3.1.1). Thus, both plants and nitrifying organ-
isms compete with heterotrophic soil organisms for the limited pool of mineral
N in soil. The balance between microbial mineralization and immobilization is
determined primarily by the C:N ratio of the substrate (section 2.3.1.2). The crit-
ical C:N ratio below which net mineralization occurs is 20-25. This figure is used
to determine the supply of soil N to grapevines by mineralization (chapter 5).
In addition to the NH
4
supply, soil temperature, moisture, pH, and O
2
sta-
tus influence the growth and activity of nitrifying organisms in the following ways:
• The optimum temperature is between 30 and 35°C, and nitrification is
very slow at temperatures
5°C.
• The optimum soil water content is about 60% of field capacity. Prolonged
desiccation kills the bacteria, but sufficient bacteria survive short periods for
the rate of nitrification to increase rapidly during the flush of
decomposition that follows the rewetting of an air-dry soil.
•
Nitrosomonas
is more sensitive to low pH than
Nitrobacter
. But
measurements of the short-term nitrification rate at nonlimiting NH
4
concentrations indicate that soil nitrifiers adapt to the prevailing pH, so
