The Soil Biology Primer
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. (See figure) 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 (Figure 3), 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 (Figure 4) are a type of
endomycorrhizal fungi. 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.
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Figure 1: 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 |
Figure 2: 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. |
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Figure 3: 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 |
Figure 4: The dark, round masses inside
the cells of this clover root are vesicules for the arbuscular
mycorrhizal fungus (AM). Credit:
Elaine R. Ingham |
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.
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Figure 5: 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. |
Figure 6: 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 |
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.
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Figure 7: Mycorrhizal fungi link root cells to soil
particles. In this photo, 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. |
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