Soil Ecology Background Information

Read Plaster (1997), chapter 7

Important terms for this lab:

actinomycetes
arthropods
denitrification
ecosystem
fungi
immobilization
mineralization
nitrification
nitrogen fixation
Nitrogen cycle

"We live on the rooftops of a hidden world. Beneath the soil surface lies a land of fascination, and also of mysteries, for much of man's wonder about life itself has been connected with the soil. It is populated by strange creatures who have found ways to survive in a world without sunlight, an empire whose boundaries are fixed by earthen walls."
-- Peter Farb. 1959. Living Earth. Harper and Brothers Publishers, New York.

Soil is not a sterile homogeneous material. Soil in its natural state is a living environment, teeming with biological activity functioning in a complex ecosystem. The types of life found in the soil are many, and the numbers of organisms are enormous. Plaster (1992) estimates that one-fourth teaspoon of a fertile soil (about one ml) contains:

50 nematodes
62,000 algae
72,000 protozoa
111,000 fungi
2,920,000 actinomycetes
25,280,000 bacteria

A handful of such soil may also contain several earthworms and many plant roots, insects, mites and spiders, and perhaps an indignant gopher or prairie dog.

Organisms serve a variety of functions in the soil ecosystem. Some of them are:

Recycling of energy and carbon - The decomposition of organic matter releases stored energy and produces carbon dioxide, which can then be reused by plants through photosynthesis.
Recycling of plant nutrients - The decomposition of organic matter transforms plant nutrients into available forms which can be taken up by plants.
Disease transmission and control - Some organisms cause or transmit diseases while others, such as Penicillium, produce substances which control disease organisms.
Nutrient/water absorption - Organisms such as fungi and actinomycetes form associations with plant roots and aid the plant in exploring the soil environment for water and nutrients.
Nitrogen fixation - Some organisms take nitrogen from the atmosphere and transform it into organic forms which are then utilized by other organisms in the ecosystem. Such organisms include bacteria in symbiosis with legume plants, blue-green algae, and some actinomycetes.
Soil mixing - Organisms that burrow through the soil mix it through their digging. Subsoil is brought up and mixed with topsoil, recycling the clays and minerals which had eluviated down out of the rooting zone.
Decomposition of chemical materials - Pesticides, petroleum products, and other organic chemicals are degraded by soil microbes; first into other organic materials, ultimately into carbon dioxide and water. Materials which resist microbial action may accumulate and lead to environmental problems.
Microorganisms and organic matter produced by soil organisms are important in the formation of soil structure. Humus and exudates of roots and microorganisms act as a glue, holding soil particles together. Fungal hyphae bind particles together. The surfaces of bacteria carry a net-negative charge, similar to clays, and have a similar effect on soil structure.

Soil organisms can be classified according to the method in which they obtain energy and carbon. Organisms are either autotrophs that obtain energy and carbon from inorganic sources, or heterotrophs that obtain energy and carbon from organic sources. Autotrophs get energy and carbon from either photosynthesis (photoautotrophs) or from chemical reactions (chemoautotrophs). Heterotrophs get energy and carbon from living plants or animals (consumers) or from dead organic matter (decomposers).

Autotrophs (energy and C from inorganic sources)
- Photoautotrophs (energy and C from photosynthesis)
- Chemoautotrophs (energy and C from chemical reactions)
Heterotrophs (energy and C from organic matter)
- Consumers (energy and C from living organisms)
- Decomposers (energy and C from dead organic material)

Soil organisms are often grouped into five classes:

animals
plants
bacteria
fungi
actinomycetes

Animals include earthworms, arthropods (insects, spiders, centipedes, etc.), and rodents (moles, gophers, prairie dogs, etc.). Earthworms have an enormous effect on the soil through their mixing of soil material and by ingesting organic matter. The organic matter remaining n their casts is partially digested, so the nutrients in it are more available to plants. Termites digest large amounts of organic matter, some of which is later available to plants, and also affect soil physical properties. Other arthropods are predators, feeding on other organisms. Nematodes are often predators in the soil, feeding on other microorganisms. Many of them have a detrimental effect on plant growth. Microscopic protozoa graze on bacteria, moving within thin films of soil water. When water or food are limiting they encyst, waiting until conditions improve.

Plants have major effects in the soil environment by the cycling of water and nutrients, production of organic matter, formation of pores by root growth, secretion of root exudates which promote soil aggregation, and production of organic acids which influence nutrient availability. Bacteria and actinomycetes that fix atmospheric nitrogen use root exudates for energy; root nodules provide a home for some of these microorganisms. Blue-green algae also fix nitrogen; especially in wet environments such as rice paddies and wetlands.

Bacteria have perhaps the greatest influence in the soil environment of all organisms. Species are either autotrophic or heterotrophic. They perform many functions in the ecosystem, most importantly in enzymatic transformations. They are able to oxidize or reduce many chemical elements in the soil. For example, nitrogen is tansformed from ammonium to nitrite and nitrate by two different species. Fixation of nitrogen from atmospheric N₂ into organic forms is also performed by bacteria, either in symbiotic associations with plants or independently. The net-negative electrical charge on bacteria and organic materials produced by them are important in the formation of soil aggregates and soil structure.

Actinomycetes are a special type of bacteria. They are single-celled and small, but they have filamentous structures (mycelia) like fungi. They have important roles in the decomposition of organic matter, release of plant nutrients, and in some cases form symbiotic relationships with plants including nitrogen fixation. Many of our important antibiotics (all the mycin compounds such as streptomycin) were first discovered through the study of actinomycetes in the soil. Streptomyces spp. are responsible for the typical odor of a plowed field.

Fungi are very important in the soil ecosystem. The decomposition of organic matter releases plant nutrients into the ecosystem for other organisms to use. Some fungi form mycorrhizal associations with plant roots - plants provide the fungi with food (sugars and other root exudates) and the mycorrhizae provide the plants with enhanced availability of plant nutrients (P,Zn,Ca,Mg,Mn,Fe, and Cu). Some fungi are important in other ways: mushrooms are important food products; many molds, like Penicillium, produce useful materials such as antibiotics. Fungal hyphae also play a role in the developement of soil structure.

Lab Study Questions

Study Questions Answers

The five classes into which soil organisms are often grouped are:

animals	earthworms, arthropods, rodents, nematodes...
plants	grass, legumes, bluegreen algae...
Bacteria	Rhizobium, Azobacter, Nitrosomonas, Nitrobacter...
Fungi	mushroom, molds, Penicillium
Actinomycetes	streptomyces spp...

Promote Growth	Inhibit Growth
soil drainage & aeration	pesticides
compost	soil erosion
barnyard manures	industrial sewage
fertilizer	monoculture
balanced irrigation	fertilizer
erosion control	compaction
mulching	excess tillage
lime	soil sterilization

Humus and root and microorganism exudates act as glue - hold soil particles together. Fungal hyphae bind particles together and bacteria may add to net negative charge to aid in binding particles.
Crop rotation would encourage the most active and diverse soil ecosystem. Factors encouraged by this include good structure, organic matter content, optimum pH, high fertility, good porosity, etc.
Earthworms need organic matter, aeration, good moisture (not excessive, not dry), and absence of toxic chemicals.
A healthy soil ecosystem would be promoted with a management plan that included crop rotations, animal manure applications, low levels of applied chemicals, tillage methods that encouraged good structure, soil and water conservation, etc.

Exam Questions from Labs

Which of the following would promote earthworm populations?
a. mulching the soil surface
b. removing all crop residue from the field
c. treating the soil with insecticides
d. keeping the soil acidic
e. all of the above
Which of the following agricultural practices would be most detrimental to soil organisms?
a. draining a somewhat poorly drained soil
b. legally acceptable application of sewage sludge to a soil
c. driving over the soil with heavy equipment
d. applying a complete fertilizer to the soil
e. applying barnyard manure to the soil
Which of the following would you expect to be most numerous in an acre of soil?
a. groundhogs
b. fungi
c. bacteria
d. algae
e. nematodes

A really neat site with some videos of soil organisms can be found at: Soil Life Movies Check it out!

For more information on soil ecology and soil organisms, visit The Microbe zoo , making sure to stop at Dirtland!

Interesting earthworm web sites are The Lowly Earthworm: The Gardener's Friend , Building your Soil , and The Earthworms as Fertilizer Factory .

For additional information on the fascinating nematodes, check out Nematodes in General and Basic Information about nematodes .

REFERENCES

Plaster, E. J. 1997. Soil Science and Management (3rd Ed.). Delmar Publishers, Inc., Albany.

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