Agriculture Reference
In-Depth Information
Table 5.1
Organic Compounds and Their Relative Ease of Decomposition under
Aerobic Conditions
Compounds and their location
Ease of decomposition
End products
Sugars and starch, present in
cell contents
Very easy; large
k
values
CO
2
, H
2
O
Proteins, present in cell
contents and membranes
Very easy; large
k
values;
less easily decomposed
if complexed with
polyphenols
NH
4
+
and SO
4
2−
ions, CO
2
,
H
2
O
Organic acids including
RNA and DNA, in cell
contents
Easy to moderately easy;
intermediate
k
values
CO
2
, H
2
O, NH
4
+
,
H
2
PO
4
−
ions
Cellulose, cell wall materials
Moderately easy to
difficult, depending on
the organisms present;
intermediate
k
values
Humus
a
, CO
2
, H
2
O
Fats and oils (lipids), cell
contents and membranes;
waxes on leaf surfaces
Difficult to very difficult;
small to very small
k
values
Humus, CO
2
, H
2
O,
H
2
PO
4
−
(from
phospholipids)
Lignin and hemicellulose
(cell walls); polyphenols
(cell contents); chitin
(insect exoskeletons,
fungal cell walls)
Difficult to very difficult;
small to very small
k
values
Humus, CO
2
, H
2
O, NH
4
+
Note.
CO
2
= carbon dioxide; H
2
O = water; NH
4
+
= ammonium; SO
4
2−
= sulfate;
H
2
PO
4
−
= orthophosphate.
a
Dark brown to black amorphous material formed after successive cycles of organic matter decomposition.
Second, because the dimensions of
k
are time
-1
, the entity 1/
k
is a time, which
defines the turnover time (in years) for C in that soil. Thus if the average
k
value for
soil C decomposition is 0.05 units per year, the turnover time is 20 years, which
is theoretically the time taken for all the C in the soil to be replaced by incoming
C. In reality, some of the fresh residues may last only a few weeks, whereas other C
compounds that are the end product of many cycles of decomposition may last for
1,000 years or more. Nevertheless, a soil with a C turnover time of 20 years or less
is considered to be biologically active.
In a vineyard, the soil C equilibrium implicit in equation 5.2 will change if
the values of
A
or
k
change. For example, rotary hoeing the mid-rows will acceler-
ate decomposition and lead to an overall decrease in SOM. Conversely, having a
cover crop of permanent grass or a mixture of herbaceous species will decrease
k
and increase
A
, leading to a gradual increase in SOM. The effect of cover crops and
the addition of various kinds of organic matter to vineyard soils are discussed in
“Changes in Soil Organic Matter,” this chapter.
