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Mycorrhizal fungi - powerhouse of the soil

Mycorrhizal fungi - powerhouse of the soil

Christine Jones, Amazing Carbon, Ph: (02) 6772 5605
Evergreen Farming (08) 6272 454

The soil foodweb of microflora and microfauna constitutes
an underground engine of fundamental signi?cance to
plant productivity. Mycorrhizal fungi play a key role in the
functioning of this foodweb, drawing down atmospheric CO2
as dissolved organic carbon (DOC) and providing much needed energy for the soil ecosystem. Mycorrhizal fungi also
improve aggregate stability, enhance soil structure, build stable
soil carbon, improve plant water use ef?ciency and increase the
ef?ciency of utilisation of important nutrients like phosphorus,
sulphur and nitrogen.

Agricultural research tends to focus on conventionally
managed crop and pasture lands where loss of diverse perennial
groundcover and/or intensive use of agrochemicals, have
dramatically reduced the number and diversity of soil ?ora and
fauna, including bene?cial microbes such as mycorrhizal fungi.
As a result, the potential contribution of microbial symbionts to
agricultural productivity has been greatly underestimated.

What are mycorrhizae and how do they work?

Vesicular arbuscular mycorrhizae (VAM) are ‘obligate fungal
symbionts’, meaning they must form an association with living
plants. They acquire their energy in a liquid form, as dissolved
organic carbon, siphoned directly from actively growing roots.
Mycorrhizal fungi cannot obtain energy in any other way. They
have mechanisms enabling them to survive while host plants
are dormant but cannot survive if host plants are removed.
Mycorrhizal fungi produce thin, hair-like threads of cytoplasm
(hyphae) with a hyphal tip at each end. One tip enters a plant
root and the other tip explores the soil matrix. Although the
hyphae are small in diameter (usually less than 10 ?m), the
mycelial network can extend across many hectares.

Mycorrhizal fungi have a fan-shaped architecture, with
long runner hyphae branching into networks of narrower and
narrower absorbing hyphae. There can be over 100 hyphal tips
at the end of each runner. These networks extend from the root
system into the bulk soil, well beyond the zone occupied by the
roots and root hairs. The absorptive area of mycorrhizal hyphae
is approximately 10 times more ef? cient that that of root hairs
and about 100 times more ef? cient than that of roots.

An amazing symbiotic relationship

Plants colonised by mycorrhizal fungi can grow 10-20% faster
than non-colonised plants, even though they are ‘giving away’
up to 40-50% of their photosynthate to support mycorrhizal
networks (photosynthate is the soluble carbon the plant ? xed
from CO2 and sunlight). One of the reasons for this apparent
paradox is that plants colonised by mycorrhizae exhibit higher
leaf chlorophyll contents and higher rates of photosynthesis
than non-colonised plants. This enables them to ?x greater
quantities of carbon for transfer to fungal hyphae in the soil.

In exchange for soluble carbon from their host, mycorrhizal
fungi supply nutrients such as phosphorus, zinc, calcium,
boron, copper and organic nitrogen. It’s an amazing symbiotic
relationship. Mycorrhizal hyphae have a tubular vacuole
system that allows bidirectional ? ow. That is, dissolved organic
carbon from the host plant and nutrients from the soil, can
move rapidly and simultaneously in opposite directions.

All groups of mycorrhizal fungi require a living host, but
there’s more to it than just plants and fungi. A wide range
of associative micro?ora are also involved. For example,
colonisation of plant roots by mycorrhizae is enhanced by
the presence of certain ‘helper’ bacteria. There are also active
colonies of bacteria on the hyphal tips, producing enzymes
which solubilise otherwise unavailable plant nutrients.

Mycorrhizae and soil carbon

Glomalin, a long-lived glycoprotein (protein containing plant
sugar) is a highly stable form of soil carbon that provides
a protective coating for the hyphae of mycorrhizal fungi.
Networks of fungal hyphae also provide an important ? rst step
for the polymerisation of dissolved organic carbon, ultimately
leading to the formation of humus, a high molecular weight
gel-like substance that holds four to twenty times its own
weight in water. Humic substances signi? cantly improve soil
structure, porosity, cation exchange capacity and plant growth.

Both glomalin and humus are of signi? cance to the current
debate on soil carbon transience, as these stable soil carbon
fractions cannot be lost from soil during droughts or ? res.
Marie Spohn from the Universität Oldenburg has identi? ed
mycorrhizae (and the glomalin they produce) as the primary
soil carbon stabilisation mechanism in sandy soils. Previously,
soil scientists have considered carbon sequestration potential to
be constrained by the soil’s clay content. The new ? ndings are
good news for WA farmers, opening the way for much greater
levels of carbon increase in agricultural soils than previously
thought possible.

Land management impacts

Increasing the amount of stable carbon stored in agricultural
soils via mycorrhizal fungi will require a redesign of many
current land management techniques. Factors negatively
impacting on mycorrhizae include lack of continuous
groundcover, single species crops and pastures (monocultures) and application of herbicides, pesticides or fungicides.

Mycorrhizal fungi are also inhibited by the application of large
quantities of water-soluble phosphorus and by the presence
of non-mycorrhizal crops (such as canola). Tillage has a less
detrimental effect than previously assumed. Recent studies
have shown that the use of chemicals is more harmful than
moderate soil disturbance. Biology friendly farming practices
based on living plant cover throughout the year (eg cover
cropping or pasture cropping) and the use of biofertilisers,
enhance mycorrhizal abundance and
diversity and are more bene?cial for
soil health than chemical farming
systems based on intermittently bare
soils and minimal soil disturbance.

Due to their low abundance in annual based or conventionally managed
agricultural landscapes, the important
role of mycorrhizal fungi in nutrient
acquisition, plant-water dynamics
and soil building processes has been
largely overlooked.

The types of fungi that tend to survive
in conventionally managed soils are
non-mycorrhizal, that is, they use
decaying organic matter such as crop
stubbles, dead leaves or dead roots as
their energy source rather than being
directly connected to living plants.
These non-mycorrhizal fungi have
relatively small hyphal networks.

Mycorrhizae and water

It is well known that mycorrhizal
fungi access and transport nutrients
in exchange for the carbon from the host plant. What is less
well known is that in seasonally dry, variable, or unpredictable
environments (ie most of Australia), mycorrhizal fungi play an
extremely important role in plant-water dynamics. The hyphal
tips are hydrophilic (both the end in the plant and the end in the
soil) enabling both water and nutrients to diffuse from one end
to the other along a moisture gradient.

Mycorrhizal fungi can supply moisture to plants in dry
environments by exploring micropores not accessible to
plant roots. They can also improve hydraulic conductivity by
bridging macropores in dry soils of low water-holding capacity
(such as sands).
Further, mycorrhizal fungi can increase drought resistance by
increasing the number and depth of plant roots.

Perennial grasses and mycorrhizae

Higher densities of mycorrhizal hyphae are found in healthy
perennial grasslands than in any other plant community. It has
been estimated that the hyphae in the top 10 cm of four square
metres (4m2
) of perennial grassland, if joined end to end, would
stretch all the way around the equator of the earth.
Broadacre cropping could bene?t
enormously from widely spaced rows
or clumps of long-lived perennial
grasses and/or mycorrhizal fodder
shrubs. As yet we do not know the
required critical mass to improve
soil ecosystem function, but it might
only need to be 5-10% perennial
cover. In diverse plant communities,
mycorrhiza compatible plants join
common mycelial networks called
guilds. These networks connect
plants with each other, enabling
exchange of nutrients and water. This
may help explain why mixed plant
communities often perform better than
monocultures.

In addition to the resilience conferred
by mycorrhizal guilds, the bene?t
of permanent mycelial networks in
terms of aggregate stability, porosity,
improved soil water holding capacity,
reduced erosivity and enhanced
nutrient availability in soils are
immense.

Soil bene?ts in many ways from the presence of living plants
year-round, due to reduced erosion, buffered temperatures,
enhanced in? ltration and markedly improved habitat for soil
biota. Signi?cantly, it is the photosynthetic capacity of living
plants (rather than the amount of dead biomass added to soil)
that is the main driver for soil carbon accumulation.

Management techniques that improve the vigour of
groundcover, foster mycorrhizal colonisation, increase
glomalin production and enhance the humi?cation process, will
contribute to long-term carbon storage, improved soil function
and markedly increased resilience to climatic variability.