Agriculture Reference
In-Depth Information
Perennial plants in natural ecosystems are likely to differ markedly from the cultivated
annual species which have been mainly studied to date. There is good evidence that
the patterns of root distribution and nutrient acquisition strategies differ between species.
Such differences extend to the chemical properties of their exudates and mucilages
and to their microbial and microfaunal communities. It is likely that the rhizosphere
processes operating in species that are unlike in such fundamental ways, will also differ
and provide a basis for their functional classification.
Within a single plant species, phenological and seasonal climatic change may also
lead to a different balance of processes as root activity varies. Billes and Bottner (1981)
distinguished two seperate effects in the rhizosphere of wheat comprising:
(i) living rhizosphere effects from germination to flowering and characterised by an active
production of exudates and mineralisation of nitrogen from soil organic matter; and
(ii) dead rhizosphere effects that occur during root senescence and decomposition, with
nitrogen immobilisation occurring during decomposition of dead roots with high
C:N ratios.
3.2
Rhizosphere processes
3.2.1
NUTRIENT UPTAKE FROM THE SOIL
Nutrient uptake by roots is not normally constrained by their capacity to absorb
nutrients, but rather by their limited capacity to explore sufficient volumes of the dilute
soil solution, and to mobilise mineral nutrients associated with the soil solid phase.
Two general processes allow roots to use nutrient sources other than those contained
in the soil solution they make contact with. These are: (i) the rhizosphere interactions
that permit nutrient mobilisation from mineral pools and soil organic matter reserves
and (ii) the exploitation of plant nutrients in soil solutions distant from the roots through
associations with mycorrhizal fungi.
3.2.1.1
Mobilisation of nutrients from soil organic matter:
the role of microfoodwebs
The priming of microfloral activity in the rhizosphere by root exudates results in
the mineralisation and assimilation of nutrients from the soil organic matter pool by
free-living micro-organisms (Trofymow and Coleman, 1982; Clarholm, 1983). Death of
these micro-organisms may result from energy shortages, predation by protists and
nematodes or attack by bacteriophage viruses leading to the transfer of some portion of
their contained nutrients to plants. These processes operate at single microsites, in which
microfloral and microfaunal communities progressively alter during root growth
(Figure IV.38). The mucilage produced in the root-cap region alters the soil physical
structure and the energy available at the microsite:
