available to the host. As nitrogen is depleted, the
non-legumes are excluded and the legumes estab-
lish on the basis of a regular nitrogen supply
provided by symbiotic rhizobia (Fig. 13.3). Ensur-
ing the establishment of legumes and the exclusion
of non-legumes may have a selective advantage for
the bacterial symbionts as this would ensure a
ready supply of photosynthetic carbon from the
legume hosts. However, suitable rhizobia are not
always available (see Parkeret al. 2006). van der
Heijdenet al. (2006a) have demonstrated in micro-
cosm experiments that legume biomass decreases
in the absence of rhizobia. In these experiments, the
non-legume biomass was largely unaffected by rhi-
zobia. While the non-legumes mainly acquired ni-
trogen from the soil, the legumes obtained nitrogen
fixed from the atmosphere. This suggests that, in
nitrogen-limited environments, rhizobia could se-
lectively favour the establishment of their host le-
gumes. As the legumes become abundant, soil
nitrogen levels increase due to nitrogen fixation.
Grasses benefit from the increased nitrogen avail-
ability and outcompete the legumes until nitrogen
availability decreases so that legumes become more
abundant, starting the cycle again. This fluctuation
in species dominance can have important conse-
quences for stability of plant communities (Schwin-
ning and Parsons 1996).
Extensive literature is available on the legume–
rhizobia symbioses and the effects of these sym-
bioses on plant growth and nitrogen fluxes into
the rhizosphere. However, little attention has been
paid to the impacts of these symbioses on commu-
nity organization, structure and ecosystem process-
es. Further studies are needed to understand (1)
how these symbioses function in community orga-
nizations and (2) the effects of fixed nitrogen on
plant species composition at the ecosystem level.
13.3.3.2 Mycorrhizal symbioses
The symbiosis between the majority of plants and
mycorrhizal fungi is one of the most abundant and
ecologically important symbioses on Earth. Mycor-
rhizal fungi can provide resistance to disease and
drought, and supply a range of limiting nutrients
including nitrogen, phosphorus, copper, iron and
zinc to the plant in exchange for plant carbon.
Mycorrhizal fungi can forage effectively for these
nutrients because they usually form an extensive
mycelial network of fine hyphae in the soil. It is not
unusual to find over 10 m of mycorrhizal hyphae
per gram of soil (Leakeet al. 2004). Moreover, the
diameter of mycorrhizal hyphae is up to 10 times
smaller than those of plant roots, indicating that
hyphae can enter small soil pores that are inacces-
sible for plants roots. The most abundant and im-
portant groups of mycorrhizal fungi are the
arbuscular mycorrhizal (AM) fungi, the ecto-my-
corrhizal (EM) fungi and the ericoid mycorrhizal
(ERM) fungi. AM fungi are abundant in grassland,
savanna and tropical forests and associate with
many grasses, herbs, tropical trees and shrubs
(Read and Perez-Moreno 2003). EM fungi associate
with about 6000 tree species and are abundant in
temperate and boreal forests and in some tropical
forests (Alexander and Lee 2005). Ericoid mycorrhi-
zal fungi are most abundant in heathland, where
they associate with members of the Ericaceae
(Smith and Read 1997). The fungi involved in my-
corrhizal associations are phylogenetically diverse
(Jameset al. 2006), and members of several of the
major fungal clades, the Glomeromycota, Ascomy-
cota and Basidiomycota, interact with plant rootsNon-legume populationLegume population
R–R+Figure 13.3Competition between a mutualistic legume
and a non-mutualistic plant. The legumes are inferior
competitors for nitrogen extraction and in the absence of
the rhizobia mutualist (R), the legume population is
eventually excluded. In the presence of the rhizobia
mutualist (R+), the legume population is able to obtain
nitrogen and excludes the competing non-legume
population.188 FUTURE DIRECTIONS