Geoscience Reference
In-Depth Information
reduction and solubilization of metals
; the topic will be covered in
Chapter 12;
complexation
; certain macromolecules can fix metals (Chap. 11)
and even take part in contaminant transport;
association with mineral surfaces, the oxides and the hydroxides
;
organic substances are associated with various elements to form
stable aggregates; this contributes to their preservation in soils
(Chap. 10) and has consequences on the dynamics of water;
storage of elements
; soil organic matter constitutes a reserve pool
and temporary fixation of carbon, nitrogen, phosphorus, and
calcium and other basic cations. This is important for plant nutri-
tion, in soil development and also in global equilibria (soil/
atmosphere exchanges).
The various roles mentioned above will be more or less important
according to the nature, abundance and seasonal variation of the residues
and the environmental conditions.
2.5.2
Carbon Compounds and Their Evolution
Litter (leaves, needles and twigs under forest), root secretions and
living or dead soil microorganisms constitute the soil organic matter
(SOM). They are transformed in two ways: (i) oxidation reactions on
contact with oxygen of the air and (ii) biochemical transformations
related to decomposing organisms using enzymes (cellulases, proteases,
lipases and ligninolytic enzymes). A schematic diagram is given below
(Fig. 2.20).
The
labile fraction
is immediately used by the microorganisms, a
small amount as nutrition and a large portion as energy source.
Mainly, this fraction is thus mineralized and yields mostly gas-
eous products that return to the atmosphere: CO
2
, CH
4
, H
2
S, NO,
N
2
O. Others, non-volatile, are utilized by plants: NH
4
, HPO
4
2-
,
Ca
++
, K
+
, Mg
++
.
The scarcely biodegradable fraction, the
resistant macromolecules
,
will lead to formation of new compounds stable in the soil, com-
plex compounds of high molecular weight:
the humic substances
.
The microorganisms more or less consume both of them. On
dying, they contribute in turn to enriching the soil in different carbon
compounds.
The carbon cycle in the soil
