Agriculture Reference
In-Depth Information
CO
2
Metabolized C
in substrate
microbial cells
(carbohydrates,
proteins, and lipids)
Net immobilization if C:N
The influence of the C:N ratio of an
Figure 2.10
N in substrate
ratio > 20-25
organic substrate on the balance
between net mineralization or
Net mineralization
+
if C:N ratio < 20-25
NH
4
immobilization of N.
Importance of the C:N Ratio
Grapevines, like all plant material, are composed of carbohydrates, proteins, and
fats, plus smaller amounts of organic acids, lignin, pigments, waxes, and resins.
The bulk of the material is carbohydrate, of which sugars (mainly sucrose) and
starch are rapidly decomposed, whereas the structural molecules such as hemicel-
lulose, cellulose, and especially lignin are less readily decomposed. Most fats, waxes,
and resins are only slowly decomposed. Protein breakdown begins in the senesc-
ing leaves before they fall to the ground, as evidenced by the yellowing of older
grapevine leaves when the berries reach full ripeness. Complete protein breakdown
to amino acids is rapid once the leaves reach the soil. Microorganisms invade the
litter and use some or all of the amino-N to synthesize their own proteins. Whether
there is enough amino-N to meet the needs of the microorganisms depends on
the C:N ratio of the substrate, as well as on the demands for growth of the de-
composer organisms, as shown in figure 2.10. Any surplus N appears in the soil
as NH
4
ions, which are available for plant growth.
The C:N ratio of fresh litter and dung from several sources is given in table
2.3. As the organic matter passes through successive cycles of decomposition, the
C:N ratio becomes smaller. For example, the C:N ratio of
SOM
in well-drained
soils of pH
7 is very close to 10. The ratio tends to be higher in poorly drained
soils or in soils on which mor humus forms (section 1.3.3.1).
2.3.1.2
The Soil Biomass
Microorganisms are mainly responsible for the decomposition of complex organic
molecules in litter and
SOM
; they are called “
decomposers
.” However, in the de-
composition process, there is much interdependence between the microorganisms
and small animals, or the mesofauna. The digestive processes of the mesofauna
are relatively inefficient, but they physically break residues into smaller and smaller
pieces as they feed, creating a larger surface for the microorganisms to colonize.
For this reason they are called “
reducers
.” The intimate interaction among the re-
ducers, the decomposers, and the residues they feed on is sometimes called a “food
web.” Collectively, the mass of organisms in a given quantity of soil is called the
soil biomass
.
2.3.2
The Decomposers
The decomposers consist of the
microflora
(members of the plant kingdom such
as algae, bacteria, fungi, and actinomycetes) and
microfauna
(soil animals
0.2
2.3.2.1
