Agriculture Reference
In-Depth Information
environment (Barber and Martin, 1976; Prikryl and Vancura, 1980; Heulin
et al.,
1987).
Microbial activity is therefore not limited by available carbon since the concentration
of available C is several times higher in the rhizoplane than in the soil beyond the
rhizosphere (Cheng
et al.,
1994). Again, different strains and species of bacteria have
different capacities to stimulate exudation.
(iv) As root growth continues, full colony development now occurs in the elongation
zone of the root. At this stage, many microcolonies, 5 to 10 µm in diameter have been
observed, closely packed in the rhizoplane and inner rhizosphere on the roots of
Trifolium
subterraneum.
As the average carbon:nitrogen ratio of exudates is approximately 30
(Lynch, 1982), nitrogen is deficient in terms of optimal microbial activity and must be
supplemented. It may be supplied through N-fixation, and from the external soil solution
through the activity of enzymes diffusing into the mucigel. Comparable activity, although
less intense, may also occur in the rhizosphere soil.
Micro-organisms appear to be able to partly digest the organic matter of the rhizosphere
soil by priming microfloral activity with mucilage. In laboratory experiments using
labelled mucilage, Mary
et al.
(1992) measured the priming effect as the increased
soil organic matter mineralised due to the addition of the mucilage (Figure IV.40).
