Agriculture Reference
In-Depth Information
IV.3
RHIZOSPHERE
The concept of the rhizosphere was initially propounded by Hiltner (1904) to describe
the specific environment comprising the soil-root interface. The rhizosphere may be
defined as 'that narrow zone of soil subject to the influence of living roots, as manifested
by the leakage or exudation of substances that affect microbial activity' (Curl and
Truelove, 1986) or 'the sum of the microsites where interactions, often mutualistic,
develop between living roots and micro-organisms' (Bottner and Billes, 1987). The con-
cept has been recently extended to the whole volume of soil that is directly influenced
by roots, and includes all the biological, chemical and physical processes associated with
soil functioning. This definition is retained in this chapter.
Such interactions may occur within the root (endorhizosphere), outside it (exorhizo-
sphere) or at the surface of the root (rhizoplane). The term 'mycorrhizosphere' has
also been used to describe the extended rhizosphere of a root colonised by mycorrhizal
fungi (Garbaye, 1991). The rhizosphere may comprise a significant proportion of the soil
volume and be an important source of functional diversity while the mycorrhizosphere
may form a substantial extension to the rhizosphere, especially in those species with
reduced root systems.
Rhizosphere effects are most clearly expressed in the fine-root fraction where exudation
is at a maximum. Nonetheless, dead roots are an essential energy source for subterranean
organisms and provide another focus for so-called secondary rhizosphere effects.
The rhizosphere is the site of a range of complex interactions between the living and
dead soil components which may be grouped into the following three main processes:
(i) the extraction of nutrients from the soil solution, organic reserves and from the general
and soil atmospheres (N), to be later absorbed by the roots and their associated microflora
and fauna; (ii) the production of modifier substances which promote plant growth
(hormones and growth factors), mediate the level of competition with other plants
or prevent attacks by pathogenic micro-organisms (allelopathy); and (iii) the formation
and maintenance of soil macro- and microstructure. In addition, the development of
such heterotrophic microbial activities as ammonification, denitrification, methano-
genesis and sulphate reduction has great relevance to biogeochemical cycles and
environmental issues.
3.1
General characterisation
The rhizosphere system includes the following components: (i) the soil organic matter
and root litter, which provide energy and nutrient elements; (ii) living roots as macro-
organisms, which promote microbial activity; (iii) free-living micro-organisms dominated
by heterotrophic bacteria and such predators as nematodes and protists, (iv) symbiotic
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