organic matter. They absorb phosphate in excess of
requirements and store it in the form of polyphosphates,
which can then be released to the plant.
Host ranges and communities of AM fungi
The AM fungi have astonishingly wide host ranges. At
least in artificial inoculations, an AM fungus obtained
from one type of plant can colonize the roots of many
unrelated plants. Nevertheless, a major experimental
study by Van den Heijden et al. (1998) strongly sug-
gests that plants in natural communities are colonized
preferentially by different strains of AM fungi, and that
the diversity of AM fungi in a site can influence the
plant biodiversity in natural ecosystems.
One line of evidence leading to this conclusion is
shown in Fig. 13.4. Small field plots were established
with gamma-irradiated soil and sown with 100 seeds
of each of 15 plants typical of North American “old
field” systems. The plots received either no AM fungi
or mycorrhizal inoculum composed of different num-
bers of AM species (1, 2, 4, 8, or 14). After one season’s
growth the plots were harvested and assessed by
several different parameters. The results clearly show
that, as the number of AM species is increased in
the plots, so there is a corresponding increase in total
shoot biomass, total root biomass, total length of
AM fungal hyphaeon the roots, an increase in plant
species diversityand plant phosphorus concentration.
In other words, with an increase in AM fungal diver-
sity there is an increase in plant productivity and plant
biodiversity.
Another part of this study was a greenhouse experi-
ment in containers of sterilized soil, using four strains
of AM fungi that had been isolated from a calcareous
grassland. There were six treatments: (1) soil with no
AM fungi, (2–5) soils with four separate strains of AM
fungi (labeled A, B, C and D), and (6) soil with all four
strains of AM fungi. Each soil container was sown with
a mixture of 70 seeds (of 11 different plant species) col-
lected from the calcareous grassland. Over two growing
seasons the above-ground parts of each plant species
were harvested separately and the cumulative plant
biomass of each species was determined (Fig. 13.5).
The most striking feature in Fig. 13.5 is that differ-
ent species in the plant community seem to respond
differently to different mycorrhizal fungi. For example,
the grass Brachypodium pinnatumgrew poorly in the
absence of mycorrhizas and also poorly with mycorrhizal
fungus B, but better with mycorrhizal fungus C. The
grassland herb Prunella vulgaris(commonly known as
“self-heal”) also grew poorly in the absence of mycor-
rhizas but better in the presence of any mycorrhizal
fungus. The sedge Carex flaccagrew best in the absence
of mycorrhizal fungi, and very poorly in the mixture
of all four AM fungi. This is not surprising because some
families of flowering plants, including sedges (Cyper-
aceae), rushes ( Juncaceae), crucifers (Cruciferae), and
Chenopodiaceae, are typically nonmycorrhizal. They
tend to be plants of open habitats and are primary
colonizers of bare or frequently disturbed soils. They
were probably mycorrhizal at some stage but as typ-
ical open-habitat species they probably benefit from
being nonmycorrhizal.260 CHAPTER 13
Fig. 13.3Single arbuscules of AM fungi growing within root cells. The arbuscules enter individual root cells from inter-
cellular hyphae, then branch dichotomously and repeatedly so that most of the cell volume is filled with finely branched
AM hyphae, providing a large area of interface for potential nutrient exchange between the fungus and its plant host.