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Soil Biology
The Soil Biology Primer
Chapter 3: BACTERIA
By Elaine R. Ingham
THE
LIVING SOIL: BACTERIA
Bacteria
are tiny, one-celled organisms – generally 4/100,000 of an inch wide (1 µm)
and somewhat longer in length. What bacteria lack in size, they make up in
numbers. A teaspoon of productive soil generally contains between 100 million
and 1 billion bacteria. That is as much mass as two cows per acre.
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Figure 1: A ton of
microscopic bacteria may be active in each acre of soil.
Credit: Michael T. Holmes, Oregon State University, Corvallis. |
Figure 2:
Bacteria dot the surface of strands of fungal hyphae.
Credit: R. Campbell. In R. Campbell. 1985. Plant Microbiology.
Edward Arnold; London. P. 149. Reprinted with the permission of
Cambridge University Press. |
Bacteria
fall into four functional groups. Most are decomposers
that consume simple carbon compounds, such as root exudates and fresh plant
litter. By this process, bacteria convert energy in soil organic matter into
forms useful to the rest of the organisms in the soil food web. A number of
decomposers can break down pesticides and pollutants in soil. Decomposers are
especially important in immobilizing, or retaining, nutrients in their cells,
thus preventing the loss of nutrients, such as nitrogen, from the rooting zone.
A
second group of bacteria are the mutualists
that form partnerships with plants. The most well-known of these are the
nitrogen-fixing bacteria. The third group of bacteria is the pathogens.
Bacterial pathogens include Xymomonas
and Erwinia species, and species of Agrobacterium
that cause gall formation in plants. A fourth group, called lithotrophs or
chemoautotrophs, obtains its energy from compounds of nitrogen,
sulfur, iron or hydrogen instead of from carbon compounds. Some of these species
are important to nitrogen cycling and degradation of pollutants.
WHAT DO BACTERIA DO?
Bacteria from all four groups
perform important services related to water dynamics, nutrient cycling, and
disease suppression. Some bacteria affect water movement by producing substances
that help bind soil particles into small aggregates (those with diameters of
1/10,000-1/100 of an inch or 2-200µm). Stable aggregates improve water
infiltration and the soil’s water-holding ability. In a diverse bacterial
community, many organisms will compete with disease-causing organisms in roots
and on aboveground surfaces of plants.
A
FEW IMPORTANT BACTERIA
Nitrogen-fixing
bacteria form
symbiotic associations with the roots of legumes like clover and lupine, and
trees such as alder and locust. Visible nodules are created where bacteria
infect a growing root hair (Figure 4). The plant supplies simple carbon
compounds to the bacteria, and the bacteria convert nitrogen (N2) from air into a form the
plant host can use. When leaves or roots from the host plant decompose, soil
nitrogen increases in the surrounding area.
Nitrifying
bacteria
change ammonium (NH4+)
to nitrite (NO2-)
then to nitrate (NO3-)
– a preferred form of nitrogen for grasses and most row crops. Nitrate is
leached more easily from the soil, so some farmers use nitrification inhibitors
to reduce the activity of one type of nitrifying bacteria. Nitrifying bacteria
are suppressed in forest soils, so that most of the nitrogen remains as
ammonium.
Denitrifying
bacteria
convert nitrate to nitrogen (N2) or
nitrous oxide (N2O) gas. Denitrifiers are anaerobic, meaning they are active where oxygen
is absent, such as in saturated soils or inside soil aggregates.
Actinomycetes are a large group of bacteria that grow as hyphae like fungi (Figure 3). They
are responsible for the characteristically “earthy” smell of freshly turned,
healthy soil. Actinomycetes decompose a wide array of substrates, but are
especially important in degrading recalcitrant (hard-to-decompose) compounds,
such as chitin and cellulose, and are active at high pH levels. Fungi are more
important in degrading these compounds at low pH. A number of antibiotics are
produced by actinomycetes such as Streptomyces.
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Figure 3: Actinomycetes, such as this
Streptomyces, give soil its "earthy" smell.
Credit: No. 14 from Soil Microbiology and Biochemistry Slide
Set. 1976. J.P. Martin, et al., eds. SSSA, Madison, WI |
Figure 4: Nodules formed where Rhizobium
bacteria infected soybean roots.
Credit: Stephen Temple, New Mexico State University |
WHERE
ARE BACTERIA?
Various
species of bacteria thrive on different food sources and in different
microenvironments. In general, bacteria are more competitive when labile
(easy-to-metabolize) substrates are present. This includes fresh, young plant
residue and the compounds found near living roots. Bacteria are especially
concentrated in the rhizosphere, the narrow region next to and in the root.
There is evidence that plants produce certain types of root exudates to
encourage the growth of protective bacteria.
Bacteria alter the soil environment to the extent that the soil
environment will favor certain plant communities over others. Before plants can
become established on fresh sediments, the bacterial community must establish
first, starting with photosynthetic bacteria. These fix atmospheric nitrogen and
carbon, produce organic matter, and immobilize enough nitrogen and other
nutrients to initiate nitrogen cycling processes in the young soil. Then, early
successional plant species can grow. As the plant community is established,
different types of organic matter enter the soil and change the type of food
available to bacteria. In turn, the altered bacterial community changes soil
structure and the environment for plants. Some researchers think it may be
possible to control the plant species in a place by managing the soil bacteria
community.
BUG BIOGRAPHY: Bacteria That Promote Plant Growth
By
Ann Kennedy, USDA Agricultural Research Service, Pullman, WA
Certain strains of the soil bacteria Pseudomonas
fluorescens have anti-fungal activity that inhibits some plant pathogens. P.
fluorescens and other Pseudomonas
and Xanthomonas species can increase plant growth in several ways. They
may produce a compound that inhibits the growth of pathogens or reduces invasion
of the plant by a pathogen. They may also produce compounds (growth factors)
that directly increase plant growth.
These
plant growth-enhancing bacteria occur naturally in soils, but not always in high
enough numbers to have a dramatic effect. In the future, farmers may be able to
inoculate seeds with anti-fungal bacteria, such as P. fluorescens, to ensure that the bacteria reduce pathogens around the seed
and root of the crop.
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Fungi"
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