INTERNAL ENVIRONMENT, MICROCLIMATE AND RESOURCES - Soil Ecology - page 131

The particle size distribution of soil organic matter is strongly affected by land use
practices. In Martinique, ten years of intensive vegetable production in a vertisol resulted
in the loss of two thirds of the organic matter, particularly from the coarse
(loss of 78 %) and the finest fractions (Martin, 1990; Albrecht et al., 1986)

Distribution within aggregates
Organic particles may be enclosed within organo-mineral aggregates and thus physically
protected from microbial digestion. This protection appears to be most effective when
oxide minerals are abundant; their significant amounts of positive charge attract the
negatively-charged humic molecules. At the scale of larger aggregates ( i.e., 50 to
and more), simple dilution of organic panicles in the often compact mineral
matrix makes them less susceptible to microbial attack. For example, mineralisation
of soil organic matter in the casts of geophagous earthworms is much slower than in

smaller aggregates (less than 2 mm) formed in the same soil. These casts occur as large

aggregates (>5 mm diameter) with low internal porosity which limits microbial activity

(Martin, 1989)(Chapter IV.4).

From the above, the distribution of soil organic matter particles between the different
size classes of aggregates may affect their accessibility to decomposers. The sharp
decrease in soil organic matter concentrations following cropping, especially in the
tropics, is largely correlated with a reduction in the degree of aggregation (Feller
et al ., 1995).
In most soils, the difference in particle size distributions after (a) gentle separation of
aggregates (by dry-sieving or gentle shaking in water) and after (b) total disruption by
sonication (use of ultrasound energy) provides a measure of the effective strength of
aggregates. Protection of soil organic matter within aggregates depends on both the
degree of aggregation and the resistance of these aggregates to dispersion by water or
more powerful forces.
Scheu et al. (1996) found considerable increases in microbial respiration following
removal of protection by the disruption of aggregates collected from three temperate-
climate forested soils. The effect was most marked in the larger aggregates and in the
most clay-rich of the three soils sampled.

Temporal variation of soil organic matter quality
When ecosystems are in an approximate state of equilibrium, the amounts and qualities
of soil organic matter vary little with time. However, in situations of strong seasonality
and rapid soil organic matter turnover, the quality of the soil organic matter may differ
seasonally. This occurs, for example, in sandy alfisols in the moist savannas of the Côte
d'Ivoire where the nutritional value of the soil for geophagous earthworms is greatest at
the end of the rainy season and lowest a few weeks after the onset of rains (Lavelle and
Meyer, 1983). Microbial respiration follows a similar temporal pattern in these soils
(Abbadie, personal communication).
At longer temporal scales, changes in the vegetation cover are reflected in different
soil organic matter characteristics. Table I.30 illustrates the changes measured in a West
African humid savanna due to an increased dominance by trees following protection
from fire (Martin et al., 1990). While only slight differences in carbon concentrations are