FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 440

substantial: for example, Dommergues (1997) reports that Casuarina equisetifolia may
fix from 15 to 94 kg ha although this high potential may be limited by unfavourable
soil, climatic and other factors. N-fixation rates are also highly variable between species
and even between clones of such host plants as Casuarina equisetifolia grown in West
Africa (Sougoufara et al., 1987).
A number of symbiotic N-fixing associations occur between non-leguminous plants and
bacteria of the genera Azospirillum, Azotobacter, Acetobacter, Azoarcus and others (Baldani
et al., 1997) and more are likely to be found. These associations have been mainly described
in graminaceous species although they also occur in palms and other woody taxa; some
are highly specific to particular hosts. They do not involve the formation of nodules.
Three basic groups of non-nodulating N-fixing associations are known, depending
where the association occurs in the plant and on the properties of the organisms involved.
The rhizosphere organisms survive well in soil and colonise the root surface. An example
is the highly specific association between Azotobacter paspali and Paspalum notatum
cv batatais which, on an areal basis, has been estimated to fix ca. 20 kg (Boddey
et al., 1983). Endophytes are organisms that may colonise the internal tissues of their
host plants. The facultative endophytes survive well in soils and colonise the rhizosphere
and root interior; they are mainly Azospirillum species. Yield increases from these asso-
ciations are undoubted although the modes of action have been questioned. Apart from
N-fixation, yield increases may also be due to the stimulation of plant growth by phyto-
hormone production or an improved capacity of the plant to metabolise assimilated N.
Obligate endophytes do not survive well in soils and colonise the internal plant tissues,
including roots, stems and leaves. Their hosts include many plants of economic importance,
including sugarcane and other graminaceous species, coffee and oil palms.
Fixation in non-leguminous plants may make an important contribution to total
N inputs in tropical grasslands, as shown above ( e.g., Robertson and Rosswall, 1986).
In Guinean savannas at Lamto (Côte d'lvoire), Balandreau and Villemin (1973) estimated
non-symbiotic N fixation at 12 kg

However, this estimate may be an under-

estimate since it was based on the acetylene reduction technique (Boddey, 1987;

Abbadie et al., 1992).
Heterocystous cyanobacteria form N-fixing associations with a diverse range of fungi
and members of the plant kingdom. The cyanobacteria involved appear to belong
largely to the genera Nostoc, Anabaena and possibly to Calothrix and some others.
These organisms enter into N-fixing associations with fungi to form certain types
of lichens and with liverworts (Hepatophyta) and homworts (Antherophyta)
(Rodgers and Stewart, 1977; Stewart and Rodgers, 1977). They also associate with the
ca. 135 species of primitive Gymnosperms of the Cycadophyta (Grobbelaar, 1993).
Anabaena enters into N-fixing associations with the floating fern Azolla and this is
of importance in the N-nutrition of wetland rice crops where it supplies an average of
15 Mg biomass or 30 kg N to the rice crop as a green manure (Roger, 1995).
Nostoc also enters into an unusual intra-cellular association with the ca. 50 known
species of the largely tropical and southern-hemisphere herbaceous angiosperm genus
Gunnera (family Haloragaceae) (Bergman et al., 1992a). In the cycads, Nostoc is
the most common symbiont and, whereas mixtures of several strains can associate
with a given plant host, single strains appear to predominate in individual hosts.