Agriculture Reference
In-Depth Information
Organic Matter and Soil Organisms
2.3
Inputs of Plant and Animal Residues
Plant residues include aboveground litter fall (leaves and branches) and below-
ground root material. Animal residues include dead bodies and excreta. Below-
ground, root tips produce mucilage that lubricates the root as it grows through
the soil. Epidermal cells produce exudates, and as the cells mature they are abraded
and sloughed off, releasing their contents into the soil. Thus, belowground C in-
puts comprise dead root material, “sloughed-off” root cells, and C compounds ex-
uded into the surrounding soil, which is described as the
rhizosphere
. The total
quantity of C compounds released is 5-8% of the total plant C, excluding C re-
leased as respired CO
2
(Tinker and Nye 2000). The return of C in animal corpses
and excreta is obviously highly variable in type and amount. Table 2.3 gives ex-
amples of C inputs and C:N ratios of materials from different sources.
2.3.1
Mineralization and Immobilization
All organic material is a potential substrate for soil organisms, most importantly
the microorganisms that derive energy for growth from the oxidation of complex
organic molecules. During decomposition, essential elements are converted from
organic to simple inorganic forms through a process called
mineralization
. For ex-
2
2.3.1.1
ample, organically combined N, P, and S appear as NH
4
, H
2
PO
4
, and SO
4
ions, and more than half the C is released as CO
2
. The remaining substrate C is
synthesized into microbial cell substances that form the
microbial biomass
. A vari-
able proportion of other essential elements such as N, P, and S is also incorpo-
rated, making these elements unavailable for plant growth until the organisms die
and decay. This process is called
immobilization
. The residues of the organisms,
together with the more recalcitrant parts of the original substrate, accumulate in
the soil as
soil organic matter
(
SOM
).
Annual Plant Litter and Animal Returns and C:N Ratios
Table 2.3
Organic C
a
(t/ha)
Source
C:N Ratio
Coniferous forest
1.5-3
90
Deciduous forest
1.5-4
25-44
Temperate grassland
2-4
25-40
Legumes (medics and clovers)
1-2
10-20
Dung from dairy cows at a high stocking rate
1.7-2.3
20-25
Arable farming (cereal straw and stubble)
1-2
40-120
Chenin Blanc vines at 1680/ha
leaf fall and prunings
1.2
22
Cabernet Sauvignon vines
leaves
1
26-44
2-year-old wood
—
80
a
For an approximate conversion to organic matter, multiply these figures by 2.5.
Source:
Data from Williams and Biscay (1991), Mullins et al. (1992), and White (1997)
