FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 379

by free-living micro-organisms‚ principally fungi‚ together with fragmentation and
digestion by invertebrates. In general‚ an important part (10 to 20 %) of the minerals
released and water-soluble elements are eluviated downwards into the mineral soil. They
may then be absorbed by deeper soil roots‚ their mycorrhizal fungi or the free-living
microflora; they may also be insolubilised or adsorbed onto the surfaces of soil col-
loids.
All the non-predatory 'epigeic' fauna ( i.e.‚ invertebrates that feed on and live in the
surface litter‚ Bouché‚ 1977) participate in fragmentation: large myriapods (iulids and
polydesmids)‚ isopods and epigeic earthworms are the main macrofaunal groups while
micro-arthropods and Enchytraeidae represent the mesofauna. The microfauna is also
represented in the litter system‚ but is relatively unimportant‚ except in extremely arid or
cold environments.

1.4.3 THE RHlZOSPHERE

The rhizosphere is defined as that part of the soil which is influenced by roots (see
Chapter IV.3). Soil organic matter resources are exploited through active mutualistic
relationships with that section of the soil microflora that is associated with living roots.
These relationships are mediated by root exudates and regulated by the micropredator
food-web (protists and nematodes with bacteria as prey) (see Section IV.1.5.3). In this
system‚ roots regulate microbial activity by providing readily-assimilable carbon sources
(rhizodeposited materials) which stimulate the activities of the free-living soil microflora
and the community of obligate (or occasionally facultative) microbial symbionts.
Rhizodeposition may represent from 5 to 30 % of total photosynthates i.e.‚ several
Mg (Hale et al.‚ 1981; Trofymow et al.‚ 1987). Micro-organisms initially develop
utilising exudates although they also digest the more complex substrates contained
within the soil organic matter through a priming effect ( sensu Jenkinson‚ 1966) leading
to a significant mineralisation of soil organic matter (Sallih et al.‚ 1987) (see Section
IV.1.5.2 on priming effects). These micro-organisms may directly release metabolites
that are used by the plant. However‚ much of this release may result from the activity
of micropredator food-webs which operate in the rhizosphere (Trofymow et al.‚ 1987).
These food-webs include bacteria as a food source‚ with protists and free-living
nematodes as predators. Micropredators release nutrients from bacteria through their
own metabolism or following their death which may be associated with the diminution
of food resources that occurs as the roots mature (Clarholm‚ 1985).
The rhizosphere is also characterised by the intense activity of obligate or
semi-obligate root symbionts‚ mycorrhizal and nitrogen-fixing micro-organisms
(including Rhizobia and Frankia ) which play substantial roles in ecosystem processes
through the provision of nutrients to their host plants.

1.4.4 THE DRILOSPHERE

The drilosphere (from the Greek 'drilos'= earthworm) is defined as that part of the soil