Agriculture Reference
In-Depth Information
At least some soil macro-organisms appear to stimulate microbial activity by producing
water-soluble organic molecules and these may be considered as the mediators which
activate mineralisation processes wherever and whenever they are produced. This activa-
tion may be intense and microbial activity eight times greater than that of a wet sieved
control soil has been measured in the gut of tropical geophagous earthworms (Barois and
Lavelle‚ 1986). From this‚ it appears that roots and invertebrates are important regula-
tors of soil organic matter decomposition in the rhizo- and drilospheres‚ through their
capacities for initiating priming effects. Priming is a key process by which these macro-
organisms may enhance microbial activity within their BSR's.
The addition of ecological mediators to soils induces highly specific effects on the
structure and activity of microbial communities. Earthworm intestinal mucus initiates
a massive microbial activation within 45 minutes whereas similar effects are only
obtained 15-30 days after root mucilage is added to the soil (Lavelle and Gilot‚ 1994;
Mary
et al.‚
1992). These processes therefore appear to operate at time scales compatible
with other activities. For example‚ the digestion that occurs during a one-hour
transit of soil material through the earthworm gut in the first case‚ and activation of the
rhizosphere microflora along the tip of a relatively slowly growing root in the second
case (see details of these processes in Sections 3 and 4 of this chapter).
1.5.3
MICRO-FOODWEBS
The vast majority of soil invertebrates rely on microbial activities to digest the organic
resources available in the soil system (see Chapter III.4.4.1‚ Figure III.61). The nature of
the relationships that particular invertebrate groups develop with the microflora depends
largely on their size. Large invertebrates interact with elements of the microflora within
structures that they create including faecal pellets‚ gallery walls and their own gut
contents. Within such structures‚ the relationships are predominantly mutualistic
although evidence exists for some degree of invertebrate predation on the microflora.
The smallest invertebrates‚
i.e.
protists and nematodes are mainly predators of
micro-organisms since their small sizes prevent them developing internal mutualistic
relationships or utilising 'external rumen' structures. Micro-organisms interact within
clearly identified foodwebs which may be further extended to include some larger
mesofaunal elements. With increasing size‚ the relationship between the microflora and
fauna gradually shifts from predation to mutualisms of increasing efficiency. The food
web concept becomes progressively more difficult to apply as the trophic structure
becomes increasingly 'fluid and interactive with individual species operating on several
levels which might be distinguished as trophically different' (Swift
et al.‚
1979; Wardle
and Lavelle‚ 1997).
It seems therefore both logical and practical to distinguish micro-foodwebs‚
i.e.‚
the foodwebs that links micro-organisms to their predators‚ from systems based on
mutualistic relationships between larger organisms and the microflora. Furthermore‚
micro-foodwebs are often included within the complex‚ globally mutualistic‚ interac-
tions that occur between macro- and micro-organisms and which operate at larger scales.
At the root surface‚ for example‚ micro-organisms and their micropredators appear
to play critical roles in releasing mineral nutrients available to the plant. In this case‚
