Agriculture Reference
In-Depth Information
which are constantly being degraded and renewed. Thus‚ changes in soil organic carbon
concentrations mainly affect the larger aggregates; the organic matter that stabilises the
smaller aggregates is incorporated in the spaces between the factoids and may be highly
protected and persistent.
1.3.4
DYNAMICS AND IMPAIRMENT OF SOIL STRUCTURE
As considered above‚ the types and spatial dispositions of the pores and aggregates that
comprise the soil structure at any specific time are the result of a large number of
physical‚ chemical and biological processes operating at different spatial and temporal
scales. Thus‚ because of the dynamic nature of soil structure‚ a balance exists between
the creation of aggregates and macropores and their destruction.
The physical forces leading to the shrinking and swelling of clays play an important
role in the dynamics of structure by forcing particles into close contact. They may act
differently between the seasons and years‚ depending on the climate and its variation.
Inputs of photosynthetically-derived energy as root growth and expansion vary seasonally‚
as do inputs of the organic matter that provide a source of energy for faunal activity
directed towards the creation of burrows and other biopores‚ casts and faecal pellets.
Aggregate stability is determined by the nature of the binding agents and will clearly
differ between the various aggregate size classes and between soils. In soils that possess
a hierarchy of aggregates‚ the larger aggregates may be disrupted by the decomposition
of the fungal hyphae or the fine roots that bind them. The subsequent breakdown of
the smaller more stable aggregates may result from disruption of their organic or
inorganic glues.
The important role of the macrofauna in aggregate formation in most soils (Bal‚ 1982;
van Breemen‚ 1993) is d u e to their promotion of primary binding through their
mechanical activities and intestinal mixing (Barois
et al
.‚ 1993). They also produce
macro-scale structures ranging from faecal pellets to several centimetres)
to large subterranean structures such as termite and ant nests. Earthworm faecal pellets
may accumulate to such an extent that certain soils have been called 'vermisols'
(Pop and Postolache‚ 1987). Probably the two most important faunal groups in aggregate
production are earthworms (Hopp and Hopkins‚ 1946; Marinissen‚ 1995; Blanchart
et al
.‚ 1997) and termites (Eschenbrenner‚ 1986; Garnier-Sillam
et al
.‚ 1988a; Miklos‚
1992) while such groups as enchytraeids (Didden‚ 1990; van Vliet
et al
.‚ 1993) are
probably of lesser importance.
The stabilities of these structures largely depend on the strength of the primary
bindings. Certain structures‚ such as the fresh casts of large endogeic earthworms‚
possess little structural stability although‚ on ageing‚ they become highly resistant to
mechanical breakdown. For example‚ casts that have undergone a number of wetting and
drying cycles (Blanchart
et al
.‚ 1993) may remain intact for several years. Production of
such casts may exceed yearly and the large earthworms that produce them
feed exclusively on small soil aggregates. As discussed further in Chapter IV‚ such earth-
worms do not re-ingest their own casts. Maintenance of an appropriate aggregate size
distribution therefore depends on the activities of the smaller earthworm species‚ other
invertebrates and non-biological processes to break down the large aggregates thereby
