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Soil Biology
Soil Biology Primer
![Earthworms](buttons/button121.jpg)
Chapter 4: SOIL FUNGI
By Elaine R. Ingham
THE LIVING SOIL: FUNGI
Fungi are
microscopic cells that usually grow as long threads or strands called hyphae,
which push their way between soil particles, roots, and rocks. Hyphae are
usually only several thousandths of an inch (a few micrometers) in diameter. A
single hyphae can span in length from a few cells to many yards. A few fungi,
such as yeast, are single cells.
Hyphae
sometimes group into masses called mycelium or thick, cord-like
“rhizomorphs” that look like roots. Fungal fruiting structures (mushrooms)
are made of hyphal strands, spores, and some special structures like gills on
which spores form. A single individual fungus can include many
fruiting bodies scattered across an area as large as a baseball diamond.
Fungi perform
important services related to water dynamics, nutrient cycling, and disease
suppression. Along with bacteria, fungi are important as decomposers in the soil
food web. They convert hard-to-digest organic material into forms that other
organisms can use. Fungal hyphae physically bind soil particles together,
creating stable aggregates that help increase water infiltration and soil water
holding capacity.
Soil fungi can
be grouped into three general functional groups based on how they get their
energy.
- Decomposers – saprophytic fungi – convert dead organic
material into fungal biomass, carbon dioxide (CO2),
and small molecules, such as organic acids. These fungi generally use complex
substrates, such as the cellulose and lignin, in wood, and are essential in
decomposing the carbon ring structures in some pollutants. A few fungi are
called “sugar fungi” because they use the same simple substrates as do many
bacteria. Like bacteria, fungi are important for immobilizing, or retaining,
nutrients in the soil. In addition, many of the secondary metabolites of fungi
are organic acids, so they help increase the accumulation of humic-acid rich
organic matter that is resistant to degradation and may stay in the soil for
hundreds of years.
- Mutualists
– the mycorrhizal fungi – colonize plant roots. In exchange for carbon from
the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients
(phosphorus, nitrogen, micronutrients, and perhaps water) to the plant. One
major group of mycorrhizae, the ectomycorrhizae (see third photo below),
grow on the surface layers of the roots and are commonly associated with trees.
The second major group of mycorrhizae are the endomycorrhizae that grow within the root cells and are commonly
associated with grasses, row crops, vegetables, and shrubs. Arbuscular
mycorrhizal (AM) fungi are a type of endomycorrhizal fungi (see fourth photo
below). Ericoid mycorrhizal
fungi can by either ecto- or endomycorrhizal.
- The third
group of fungi, pathogens or parasites, cause reduced
production or death when they colonize roots and other organisms.
Root-pathogenic fungi, such as Verticillium, Pythium, and Rhizoctonia, cause major economic losses in agriculture each
year. Many fungi help control diseases. For example, nematode-trapping fungi
that parasitize disease-causing nematodes, and fungi that feed on insects may be
useful as biocontrol agents.
![Tree roots and mycorrhizal fungi](images/M4_Fungi_LR_small.jpg) |
Many plants
depend on fungi to help extract nutrients from the soil. Tree roots
(brown) are connected to the symbiotic mycorrhizal structure (bright
white) and fungal hyphae (thin white strands) radiating into the soil.
Credit: Randy Molina, Oregon State University, Corvallis.
Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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![SSSA48 LR.jpg (152264 bytes)](images/SSSA48_LR_small.jpg) |
Fungus
beginning to decompose leaf veins in grass clippings.
Credit: No. 48 from Soil Microbiology and Biochemistry Slide
Set. 1976. J.P. Martin, et al., eds. SSSA, Madison WI. Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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![Ectomycorrhizae](images/ECTOMY1_LR_small.JPG) |
Ectomycorrhizae are
important for nutrient absorption by tree and grape roots. The fungus does
not actually invade root cells but forms a sheath that penetrates between
plant cells. The sheath in this photo is white, but they may be black,
orange, pink, or yellow.
Credit: USDA, Forest Service, PNW Research Station,
Corvallis, Oregon. Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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![VAM LR.jpg (209158 bytes)](images/VAM_LR_small.jpg) |
The dark,
round masses inside the cells of this clover root are vesicules for the
arbuscular mycorrhizal fungus (AM).
Credit: Elaine R. Ingham. Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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WHERE ARE FUNGI?
Saprophytic
fungi are commonly active around woody plant residue. Fungal hyphae have
advantages over bacteria in some soil environments. Under dry conditions, fungi
can bridge gaps between pockets of moisture and continue to survive and grow,
even when soil moisture is too low for most bacteria to be active. Fungi are
able to use nitrogen up from the soil, allowing them to decompose surface
residue which is often low in nitrogen.
Fungi are
aerobic organisms. Soil which becomes anaerobic for significant periods
generally loses its fungal component. Anaerobic conditions often occur in
waterlogged soil and in compacted soils.
Fungi are especially extensive in forested lands. Forests
have been observed to increase in productivity as fungal biomass increases.
![A desert landscape](images/JER_LR_small.jpg) |
In arid rangeland systems, such as
southwestern deserts, fungi pipe scarce water
and nutrients to plants.
Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range, Las
Cruces, NM. Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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![Mushroom](images/tettleshroom_LR_small.jpg) |
Mushrooms, common in forest systems, are the fruiting bodies made by a
group of fungi called basidiomycetes. Mushrooms are "the tip of the
iceberg" of an extensive network of underground hyphae.
Credit: Ann Lewandowski, NRCS Soil Quality Institute.
Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
|
MYCORRHIZAL FUNGI IN AGRICULTURE
Mycorrhiza is a symbiotic association between fungi and
plant roots and is unlike either fungi or roots alone. Most trees and
agricultural crops depend on or benefit substantially from mycorrhizae. The
exceptions are many members of the Cruciferae family (e.g., broccoli, mustard),
and the Chenopodiaceae family (e.g. lambsquarters, spinach, beets), which do not
form mycorrhizal associations. The level of dependency on mycorrhizae varies
greatly among varieties of some crops, including wheat and corn.
Land
management practices affect the formation of mycorrhizae. The number of
mycorrhizal fungi in soil will decline in fallowed fields or in those planted to
crops that do not form mycorrhizae. Frequent tillage may reduce mycorrhizal
associations, and broad spectrum fungicides are toxic to mycorrhizal fungi. Very
high levels of nitrogen or phosphorus fertilizer may reduce inoculation of
roots. Some inoculums of mycorrhizal fungi are commercially available and can be
added to the soil at planting time.
![A root coated with sand grains](images/COL-SH1_LR_small.jpg) |
Mycorrhizal fungi
link root cells to soil particles. In the photo at left, sand grains are bound
to a root by hyphae from endophytes (fungi similar to mycorrhizae), and
by polysaccharides secreted by the plant and the fungi.
Credit: Jerry Barrow, USDA-ARS Jornada Experimental Range, Las
Cruces, NM. Please
contact the Soil and Water Conservation Society
at
pubs@swcs.org
for assistance with copyrighted (credited)
images.
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