Agriculture Reference
In-Depth Information
(iii) not all BSR's are equally important in all ecosystems; they may develop mutualistic
and antagonistic (competitive) relationships. For example‚ litter systems dominate in
forest ecosystems‚ especially where anecic earthworms and termites are absent since
these animals often export litter to their own functional domain. In contrast‚ in savannas‚
important drilosphere and termitosphere components are normally present provided that
climatic constraints‚ biogeographic barriers or anthropogenic effects do not exclude or
eliminate them;
(iv) organisms of very different sizes are likely to react differently to temperature
changes. The adjustment of microbial activity to improved environmental conditions
(
e.g.‚
addition of a high quality organic substrate and/or water) is likely to be much faster
at high temperatures than at low. The mutualistic digestion system of the geophagous
endogeic earthworm
Pontoscolex corethrurus
is highly efficient at 25-27 °C but no longer
functions when the temperature is reduced to 15 °C. At this temperature‚ microbial activity
in the hindgut is only slightly greater (1.5 times) than that of control soil values and is five
times lower than that occurring at 27 °C (Barois‚ 1987); in consequence‚
P. corethurus
ceases to grow at 15 °C (Lavelle
et al.‚
1987).
Consequently‚ under the temperature conditions pertaining in soils of the humid
tropics‚ mutualistic relationships between soil micro- and macro-organisms are likely to
be much more frequent and better developed than in colder environments. The increased
importance of mutualism on the structure and functioning of food-webs is considered to be
a key issue in understanding the functioning of humid tropical soils (see Chapter III.4.4.2).
1.4.2
LITTER-SUPERFICIAL ROOTS SYSTEM
The principal energy source for this system is the above-ground litter (see Chapter IV.2).
It also comprises a dominant community of arthropods as macro-organisms‚ a microflora
dominated by fungi and an occasionally-dense mat of fine roots and fungal hyphae
(mycorrhizal and saprotrophic) which serve as a sink for the nutrients released from the
decomposing litter. In certain situations‚ the internal cycling of nutrients may be almost
closed‚ as in some Amazonian forests occurring on spodosols (Herrera
et al.‚
1978)‚
although water-soluble organic materials and mineral elements are normally leached
downwards into the underlying soil horizons. Litter may be exported to other systems
of decomposition by the anecic fauna (
i.e.‚
species that feed on litter but live in burrows
or galleries built within the soil: earthworms‚ some termites and species living at its
surface or in trees (some termites) (see Sections IV.4 and IV.5).
Direct mineralisation of litter by the root-fungal (mycorrhizal) association has
apparently been observed in certain forests on sandy‚ highly leached and acidic soils
such as the tropical spodosols of Amazonia (Herrera
et al.‚
1978; Leroy
et al.‚
1992).
Smith and Read (1997) among others‚ however‚ consider that AM fungi do not have the
enzymatic capacity to directly mineralise litter. It also occurs in the litter of plant
species involved in symbioses with ericoid mycorrhizae (Dighton‚ 1991) and in
temperate climate litters colonised by certain ectomycorrhizal fungi. In more fertile
soils‚ this system of direct cycling may be less important and the thickness of surface
root mats is correspondingly smaller. Other processes operating include direct decomposition
