Agriculture Reference
In-Depth Information
ondary roots may grow through areas that have already been activated by primary roots
thereby greatly enhancing subsequent activity.
3.1.1.2
Radial extension of the rhizosphere
Biological effects in the rhizospheres of most plants extend outwards from the root for
only a few tens of micrometres in most plants, diminishing with increasing distance from
the root surface through the inner and outer zones (Foster and Rovira, 1978).
The rhizoplane is that part of the rhizosphere, approximately 1 thick, in contact
with the root surface. Microfloral communities are usually dominated by bacteria and in
the rhizoplane of
Trifolium subterraneum,
small compact colonies about 5 to10 in
diameter sometimes form; they are surrounded by a capsule of polysaccharides different
from the mucilage produced by the root. Microbial communities have their greatest
diversity (
e.g.,
11 taxa recognised under the experimental conditions of Foster and Rovira,
1978) and density in the rhizoplane although Kiffer
et al.
(1977) observed that ectomycor-
rhizal infection of roots significantly depletes bacterial populations in the rhizoplane.
The inner rhizosphere includes the rhizoplane and extends up to 10
from the root
surface. Community diversity and density are still high in the
5
closest to the root
surfaces but start to decline steadily in the 5-10
layer, with values of one third to one
half of those in the rhizoplane.
The outer rhizosphere extends from 10 to 20 from the root surface. In the example
presented by Foster and Rovira (1978), species richness fell to two taxa and micro-
organism density to cells soil, or
ca.
a ninth less than the rhizoplane value
Nonetheless, this is still an order of magnitude greater than the highest
values for soils external to the rhizosphere ( cells at from the root).
Micro-organisms may colonise the root cortex by penetrating the epidermal tissue or
entering the superficial root tissues through occasional wounds. Such colonised cells and
decaying cortical cells constitute the
endorhizosphere.
Microbial density may be greater
in this part of the rhizosphere than in the mucilage, although turnover is much slower
(Knowles, 1978).
The zone of rhizosphere soil extends beyond the rhizosphere
sensu stricto
and may be
described as the zone in which microbial activity is modified by the diffusion of root
products and metabolites. In this zone, extending a few millimetres to centimetres from
the root surface, soil structure is modified and such decomposition-resistant materials as
cell debris and mucilage may preferentially accumulate, often associated with clays
(Breisch
et al.,
1975; Goss
et al.,
1984; Callot
et al.,
1984; Morel
et. al.,
1991; Misra
et. al,
1986; Miller and Jastrow, 1990; Dexter, 1991; Pojasok and Kay, 1990.). The
hyphae of the mycorrhizal fungi associated with the roots may extend for distances of
several centimetres from the root surface substantially enlarging the effective volume of
the rhizosphere (see Section IV.3.1.2.2).
3.1.1.3
Micro-environment of the rhizosphere
Micro-environmental conditions in the rhizosphere may differ from those of the external
medium. The atmosphere is enriched in
by root respiration, clay minerals may
