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Soil Microorganisms and their Functions
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Soil Microorganisms and their Functions
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Soil MicroorganismsAnd Their Functions
BACTERIA
Bacteria are organisms that have only
one cell and are therefore microscopic.
But don't let their size fool you! There are
anywhere from 100 MILLION to 1 BILLION
bacteria in just 1 TEASPOON of soil!!!
They are decomposers, eating dead plant
material and organisms' waste. By doing
Figure 1: A ton of microscopic bacteria Figure 2: Bacteria dot the surface of strands
this, the bacteria release nutrients that
may be active in each acre of soil. of fungal hyphae.
Credit: Michael T. Holmes, Oregon State other organisms could not access. The Credit: R. Campbell. In R. Campbell. 1985.
University, Corvallis. bacteria do this by changing the nutrients Plant Microbiology. Edward Arnold; London.
into a form that can be used. Do you P. 149. Reprinted with the permission of
know what soil smells like? Well Cambridge University Press.
actinomycetes, a unique type of bacteria,
cause that smell, and it is a good sign of
healthy soil. Actually, people have been
smelling soil for many, many years as a
way to judge if the land is good for
planting.
A FEW IMPORTANT BACTERIA
Figure 3: Actinomycetes, such as this Nitrogen-fixing bacteria form symbiotic
Figure 4: Nodules formed where Rhizobium
Streptomyces, give soil its "earthy" smell. associations with the roots of legumes bacteria infected soybean roots.
Credit: No. 14 from Soil Microbiology and like clover and lupine, and trees such as Credit: Stephen Temple, New Mexico State
Biochemistry Slide Set. 1976. J.P. Martin, alder and locust. Visible nodules are University
et al., eds. SSSA, Madison, WI
created where bacteria infect a growing
root hair (Figure 4). The plant supplies Nitrifying bacteria change
Actinomycetes are a large group
simple carbon compounds to the ammonium (NH4+) to nitrite (NO2-)
of bacteria that grow as hyphae
bacteria, and the bacteria convert then to nitrate (NO3-) – a preferred
like fungi (Figure 3). They are
nitrogen (N2) from air into a form the form of nitrogen for grasses and
responsible for the
plant host can use. When leaves or roots most row crops. Nitrate is leached
characteristically “earthy” smell
from the host plant decompose, soil more easily from the soil, so some
of freshly turned, healthy soil.
nitrogen increases in the surrounding farmers use nitrification inhibitors to
Actinomycetes decompose a
area. reduce the activity of one type of
wide array of substrates, but are
nitrifying bacteria. Nitrifying bacteria
especially important in degrading
Denitrifying bacteria convert nitrate to are suppressed in forest soils, so that
recalcitrant (hard-to-decompose)
nitrogen (N2) or nitrous oxide (N2O) gas. most of the nitrogen remains as
compounds, such as chitin and
Denitrifiers are anaerobic, meaning they ammonium.
cellulose, and are active at high
pH levels. Fungi are more are active where oxygen is absent, such
important in degrading these as in saturated soils or inside soil
aggregates.
compounds at low pH. A number
of antibiotics are produced by
actinomycetes such as
Streptomyces
, FUNGI
Fungi are organisms. They are not plants,
nor are they animals. They group
themselves into strings called hyphae.
The hyphae then form groups called
mycelium. They are less than an 1/32 of
an inch wide but can get as long as
several meters. They are helpful but
Figure 1: Many plants depend on fungi to
help extract nutrients from the soil. Tree could also be harmful to soil organisms.
roots (brown) are connected to the symbiotic Fungi are helpful because they have the Figure 2: Fungus beginning to decompose leaf
mycorrhizal structure (bright white) and ability to break down nutrients that other veins in grass clippings.
fungal hyphae (thin white strands) radiating organisms cannot. Fungi release them Credit: No. 48 from Soil Microbiology and
into the soil. Biochemistry Slide Set. 1976. J.P. Martin, et
into the soil, and other organisms get to
Credit: Randy Molina, Oregon State al., eds. SSSA, Madison WI.
University, Corvallis use them. Fungi can attach themselves
to plant roots. Most plants grow much
better when this happens. This is a good
relationship called mycorriza. The fungi
help the plant by giving it those needed
nutrients, and the fungi get food from the
plant, the same food that plants give to
humans. On the other hand, fungi can
get food by being parasites, attaching
themselves to plants or other organisms,
but for selfish reasons.
Figure 3: Ectomycorrhizae are important for
nutrient absorption by tree and grape roots. A FEW IMPORTANT FUNGI
The fungus does not actually invade root cells Decomposers – saprophytic fungi –
but forms a sheath that penetrates between Figure 4: The dark, round masses inside the
plant cells. The sheath in this photo is white, convert dead organic material into fungal cells of this clover root are vesicules for the
but they may be black, orange, pink, or biomass, carbon dioxide (CO2), and small arbuscular mycorrhizal fungus (AM).
yellow. molecules, such as organic acids. These Credit: Elaine R. Ingham, Oregon State
Credit: USDA, Forest Service, PNW Research fungi generally use complex substrates, University, Corvallis
Station, Corvallis, Oregon
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.
Figure 5: In arid rangeland systems, such as
southwestern deserts, fungi pipe scarce In addition, many of the secondary
Figure 6: Mushrooms, common in forest
water and nutrients to plants. metabolites of fungi are organic acids, so systems, are the fruiting bodies made by a
Credit: Jerry Barrow, USDA-ARS Jornada they help increase the accumulation of group of fungi called basidiomycetes.
Experimental Range, Las Cruces, NM. humic-acid rich organic matter that is Mushrooms are "the tip of the iceberg" of an
resistant to degradation and may stay in extensive network of underground hyphae.
Credit: Ann Lewandowski, NRCS Soil Quality
the soil for hundreds of years.
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