Agriculture Reference
In-Depth Information
N-fixation may be significant on an areal basis in these associations: amounts up to
18 kg have been reported for cycad-cyanobacterial associations in Australian
Eucalyptus
-dominated forests (Grobbelaar, 1993).
There is broad scope to manipulate N-fixing micro-organisms since natural selection
has often resulted in strains with an ability to survive under specific environmental
conditions, rather than fix N efficiently (Bowen, 1980). Considerable research efforts have
and are still being made to develop appropriate technologies that can be used at a field scale.
3.1.2.3
Faunal communities
Roots attract a large number of soil invertebrates. While species belonging to the macro-
fauna are too large to live in the rhizosphere and specifically adapt to this environment,
certain species can exploit roots as a food resource and become serious pests. Interestingly,
there seems to be a rather low proportion of macro-invertebrates that are adapted to feed
on either living or dead roots (
e.g.,
Athias
et al.,
1974) and only six of the 26 insect
orders are represented by active below-ground herbivores (Brown and Gange, 1990).
Certain taxa including Coleoptera larvae, Gryllotalpidae (Orthoptera) or earthworms
may ingest roots and future research using adequate methods may demonstrate that root
consumption is more widespread than currently known (Baylis
et al.,
1986; Villalobos
and Lavelle, 1990). There is also some evidence that endogeic earthworms might, at
least in some circumstances, preferably ingest rhizosphere soil and feed on exudates and
root litter (
e.g.,
Spain
et al.,
1990).
Microfauna and, to a lesser extent mesofauna, are specific inhabitants of the rhizosphere.
Amoebae are common components of the rhizosphere where they feed on bacteria
and fungi (
e.g.,
Darbyshire and Greaves, 1967; Stout and Heal, 1967; Ingham, 1981;
Chakraborty
et al.,
1983 ). They live close to aggregates and can produce long pseu-
dopodia that penetrate micropores to catch their bacterial prey (Darbyshire
et al.,
1989;
Foster and Dormaar, 1991). Flagellates and ciliates are also found in the rhizosphere,
although in lesser quantities. Free-living nematodes and micro-arthropods predate on
protists and may comprise a further level in the foodweb (Elliott
et al.,
1979; Moore,
1988; Elliott
et al.,
1988).
In general terms, the relative concentration of fauna in the rhizosphere varies
inversely with the soil organic matter content.
Root parasitic fauna
Nematodes.
A variable percentage (up to 83 %) of soil nematodes are root
feeders (see Chapter III.4.1.2). Environmental disturbance and the presence of host
plants generally stimulate the activity of this functional group (Ferris, 1993).
Complex biochemical interactions occur between phytoparasitic nematodes that
are attracted to potential food supplies by substances released into the rhizosphere.
These include volatile and gaseous compounds ( ethylene), soluble and diffusible
substances such as amino acids, aliphatic and organic acids, salts, ions and mucigel
(Cohn and Spiegel, 1991). Egg hatching is further stimulated by specific substances
produced in the rhizosphere (Jones, 1959; reviewed in Curl and Truelove, 1986; Norton,
