Jim Marra, Ph.D.

Room 260, Bloedel Hall

jimarra@u.washington.edu

543-1486

ESC 210 Introduction to Soils

Autumn 2002

Soil Organisms

Lecture Outline

A. Characteristic Features of Soil Food Webs

1. Energy pathways in soil ecosystems

2. Trophic functions of soil organisms

- Autotrophes - synthesize organic compounds (carbohydrates, lipids, proteins) from non-biological sources.

• photoautotrophes - utilize solar radiation to synthesize organic compounds from CO₂ (photosynthesis).

• chemoautotrophes - produce organic compounds by oxidizing inorganic chemicals. (sulfur oxidizing bacteria).

- Heterotrophes - consume energy in the form of organic compounds, biological sources

• herbivores - feed on living plants

• saprotrophes - feed on the remains of dead plants and organisms.

Grazers – feed on bacteria and fungi

Shredders – fragment plant litter as they feed

• predators - feed on living animals

• mutualists - derive nutrition through mutually beneficial symbiotic associations with other organisms (mycorrhizae fungi, nitrogen fixing bacteria, lichenized fungi)

B. Major Groups of Soil Organisms

- Bacteria - single celled prokaryotes

- Algae - single celled photosynthetic eukaryotes

- Protozoa - singled celled

- Fungi - single or multicellular

- Earthworms - segmented worms

- Nematodes - round worms

- Arthropods – insects, mites, spiders, springtails and many others.

- Mammals

C. Summary of Functions of Soil Organisms

1. Photosynthesis - plants, moss, lichens, algae, some bacteria

2. Decomposition - fungi, bacteria, invertebrates breakdown organic matter into inorganic

constituents.

3. Nutrient Cycling - microbes and invertebrates release nutrients from litter. Bacteria fix

atmospheric nitrogen

4. Modifying Soil Physical Properties -

- enhance soil structure, hydrology, O₂ exchange

microbial secretions bind soil particles into aggregates

channeling activities of invertebrates increase soil porosity

5. Mutualism -

- Nitrogen Fixation - symbiotic N2 fixation - bacteria and roots of plants (e.g. Red Alder,

lupines) transformation of atmospheric (N₂) to organic nitrogen

- Mycorrhizae - symbiotic association of fungi with fine roots of plants

- Lichens – green algae and fungi

- Termites – gut inhabiting protozoa digest cellulose, bacteria fix nitrogen

6. Pathogens, Pests and Parasites - fungi, bacteria, nematodes, insects

7. Biological Control – the use of biological agents to control the population of a pest or

pathogen.

8. Bioremediation – the use of biological agents to remove hazardous substances from the

environment.

Recommended Reference

Tugel, Arlene, Ann Lewandowski, Deb Happe-vonArb, eds. 2000. Soil Biology Primer. Rev. ed. Ankeny, Iowa: Soil and Water Conservation Society.

Available free online at:

http://soils.usda.gov/sqi/SoilBiology/soil_biology_primer.htm