SOIL FORMATION - Soil Ecology - page 169

2,000 years. However, from studies carried out in several other environments, much
older organic matter (from 3,000 to 12,000 years) has been found to occur in the
resistant pool (Stout et al., 1981).

Climate
Decomposition of organic matter is a combination of two contrasting processes.
The first is mineralisation, which is associated with the disappearance of organic matter
and its reduction to mineral components, and humification which leads to the formation
of highly-polymerised and decomposition-resistant compounds. Jenny et al. (1949) found
a satisfactory relationship between temperature and moisture conditions and both C
and N contents in soils of the Great Plains of the USA. It has been conventionally
considered that the high temperatures and favourable moisture conditions that prevail in
the wetter tropics would favour mineralisation over humification and therefore result in
high rates of organic matter breakdown. Since decomposition rates increase faster with
temperature than primary production, it was also considered that, despite their higher
organic inputs, soils of the tropical regions would have lower organic reserves than those
of colder areas (see e.g., Beck, 1971; Scharpenseel, 1988). However, extrapolations of
Jenny's model systematically under-estimated soil organic contents (Laudelout, 1990)
and the reality is much more complex (Greenland et al., 1992).
Sánchez et al. (1982) compared the total carbon contents of 61 randomly-chosen
tropical profiles with 45 similarly-chosen profiles from the temperate region, classified
as belonging to equivalent orders of the USDA classification (Soil Survey Staff, 1999).
These authors could find no significant difference between these two groups. Thus, it
appears that the increased inputs and decomposition rates in the tropics do not necessarily
lead to significant differences in accumulated carbon stocks. Coefficients of variation
within each group, however, ranged from 32 to 79 %, which indicates that many local
variables influence the dynamics of organic matter accumulation.

Local factors
Among the local factors that influence the accumulation of organic matter are the types
and quantities of the different clay minerals present. Some clays may form very stable
organo-mineral associations with soil organic matter. Organic colloids may be physically
protected from enzyme attack by their insertion between the clay platelets, or by adsorp-
tion onto their surfaces.
The nature and amount of inputs also helps to determine soil organic matter dynamics
and further stabilisation (Swift et al., 1979).

3.3 Translocation

Translocation may be defined as any movement of soil components within a profile that
affects soil development. Such movements may. either promote or act to inhibit or
partially reverse (pedoturbation) profile differentiation (Hole, 1961). Translocation may
result from both physical and chemical processes that involve the movement of soil
materials by such agencies as water and air flows and gravity; it may also occur through