Agriculture Reference
In-Depth Information
The types and amounts of clay minerals present may greatly influence the effects
mentioned above. Minerals with low specific surface areas and low charge densities
have relatively low overall electrostatic charge levels and are therefore less effective in
forming structural aggregates (see Chapter I). This is the case for many soils in which
kaolinite is the dominant clay mineral (Uehara‚ 1982)‚ as in the widespread ultisols and
oxisols (Table I.5) which comprise about 60 % of tropical soils (Sánchez and Salinas‚
1983). Despite this‚ in oxisols where the iron supply is high (as in soils derived from
basaltic parent materials) and organic matter concentrations are also high‚ the considerable
surface areas and reactivities of iron oxides may lead to high levels of complexation with
soil organic matter (Schwertmann
et al.‚
1986).
Physical protection in biological structures
Many common soil invertebrates produce compact organic and organo-mineral faecal
pellets which have a low internal porosity. There is some evidence that mineralisation of
plant material is depressed in the faecal pellets of some arthropods and enchytraeid
Oligochaeta‚ especially when they are small (see‚ for example‚ Toutain
et al.‚
1982;
Hanlon and Anderson‚ 1980; Bernhard-Reversat‚ 1993).
This process is particularly well developed in rounded earthworm casts. In the Lamto
savannas (Côte d'Ivoire)‚ endogeic geophagous earthworms may produce stable casts
from soils with high (>80 %) sand contents (Blanchart‚ 1992); decomposition of soil
organic matter is substantially reduced within these structures (Martin‚ 1991) (see also
development in Chapter IV.4.3).
1.3.3
RESOURCE QUALITY AND DECOMPOSITION PROCESSES
Decomposition rates are highly dependent on the chemical quality of the decomposing
resource. In analysing data from 192 different sites distributed across temperate to
tropical regions‚ Aerts (1997) found that the litter decomposition rate was highly
significantly correlated with quality‚ as assessed by the lignin to nitrogen ratio ( esti-
mated at 0.24). However‚ litter decomposition rates were much more closely related to
the actual evapotranspiration rate ( estimated at 0.46). As decomposition proceeds‚
resource quality rapidly changes because the readily-assimilable substrates are rapidly
metabolised and resistant compounds tend to accumulate (see‚ for example‚ Minderman‚
1968; Lobo
et al.‚
1974; Rapaire and Turenne‚ 1977; Martin and Haider‚ 1986).
There are three principal types of chemical limitations to decomposition processes:
an initial inhibition in recently-dead tissues due to the formation of phenol-protein
complexes‚ and those resulting from the accumulation of lignin and of resistant humic
compounds. At any stage‚ imbalances in the ratios of carbon to the other nutrient
elements present may also inhibit decomposition processes.
Phenol-protein complexes
When plant tissues die‚ polyphenolic compounds accumulated in vacuoles are released
into the cell contents. They react with the cytoplasmic proteins to form phenol-protein
complexes which impart the brown pigmentation to dead roots and leaves (Handley‚
1954‚ 1961; Toutain‚ 1987b). (Figure IV.8b). These complexes usually constitute 10 to
