SOIL FORMATION - Soil Ecology - page 160

Direct substrate alteration results from:
(i) Microbial activity in the rhizosphere (Leyval and Berthelin, 1982; Stucki et al., 1992);
(ii) Enzymatic reduction of iron (Berthelin and Kogblevi, 1974);
(iii) The direct activity of roots which may absorb and concentrate such minerals as
calcite in their cells (Jaillard, 1983), or at their surfaces.
Roots may produce some secondary silicates (Callot et al., 1992), and this over time
may influence much of the soil volume through exploration by the root system. Silica is
precipitated in substantial quantities in plant tissues and is returned to the soil in litter
as phytoliths (Section I.1.1.1.2). Also, clay minerals may be altered during transit
through the earthworm gut and by termites through manipulation in the buccal cavity
(Boyer, 1973a).
Indirect alteration is the modification of mineral structures by soluble organic
compounds which have the capacity to attack minerals and thereby promote rapid
weathering and leaching. However, products of this attack may be insolubilised through
precipitation or flocculation-coagulation processes in the presence of active minerals
(clays, active Fe cations) (see e.g., Toutain, 1974) or metabolised when microbial activity
is intense (Berthelin et al., 1979). In temperate regions, weathering and subsequent
leaching are also intense although aggressive organic compounds may be neutralised
rapidly through insolubilisation. Soil invertebrates often play a cardinal role in mediat-
ing these processes.
Soil invertebrates produce two kinds of faeces within which contrasting reactions occur:
(i) The casts of anecic earthworms (those that ingest a mixture of soil and surface
litter and inhabit galleries) are macro-aggregates of 2.5 to 10 mm in diameter and form
what are known as 'macroaggregate closed-systems', since they are a mixture of organic
and mineral elements supporting intense microbial activity. This leads to the rapid
flocculation of soluble organic compounds and largely restricts mineral weathering.
(ii) Arthropods that live in the surface litter and holorganic horizons produce small
and unstable faecal pellets. Such aggregates are composed of partly commin-
uted, but largely-untransformed organic matter with low concentrations of associated
minerals. Microbial activity in these aggregates is generally low. These aggregates are
subject to intense leaching and the aggressive organic compounds released promote
intense weathering and the loss of minerals. They form the basis of what is known as
'aggressive leaching systems'.
Therefore, under comparable conditions of climate, mineralogy and cation concentra-
tion, the structure of invertebrate communities can substantially influence prevailing
weathering patterns. Equivalents of both these systems may also be found in the rhizo-
sphere and their activities will depend on the chemical compositions of the root exudates
and root litter present.

3.1.4 THE MAJOR WEATHERING PATTERNS

Weathering transforms the original bedrock into a mixture of large, slightly-altered
mineral fragments and other materials, the soil skeleton; such materials, except through pedo-
turbation, are little subject to movement during soil formation. Smaller secondary minerals
and soluble materials (the soil plasma) result from weathering of the primary minerals