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Life in the Soil

Life in the Soil. Ch 5. Soil teems with life . . . 1 teaspoon of fertile soil could contain: 100 nematodes 250,000 algae 300,000 amoeba 450,000 fungi 11,700,000 actinomycetes 100,000,000 bacteria. Carbon Cycle. Part of the food chain . . .

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Life in the Soil

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  1. Life in the Soil Ch 5

  2. Soil teems with life . . . 1 teaspoon of fertile soil could contain: • 100 nematodes • 250,000 algae • 300,000 amoeba • 450,000 fungi • 11,700,000 actinomycetes • 100,000,000 bacteria

  3. Carbon Cycle Part of the food chain . . . organisms in the soil, including plants, participate • Producers – Plants are primary producers • Consumers – Primary and Secondary Carbon and Energy move up the food chain from plants to the highest level

  4. Detrital Food Chain Breaks down and recycles dead organisms • detritus – dead organisms or their products • decomposers – consume organic matter, release CO2, and leave humus

  5. Detrital Food Chain Releases plant nutrients that were tied up in bodies of plants and animals • Soil is the location of the Detrital Food Chain Thus . . . • Soil life functions to recycle Carbon and Nutrients

  6. Ecology of the Soil Five ecological roles of interest to soil students • Autotrophs (producers) and Heterotrophs (consumers) • Parasites • Predators • Saprophytes • Symbionts

  7. Ecology of the Soil Additional classifications: • Preferred environment . . . e.g. oxygen (aerobic), no oxygen (anaerobic) Note: for an overview of specific organisms refer to additional “Ch 11” handout

  8. Nutrient Cycling • Nutrients in living bodies or fresh O.M. are said to be immobilized • Microbes decompose immobilized nutrients through mineralization

  9. Nutrient Cycling • Immobilization and Mineralization are opposite processes • Most soil sulfur comes from the weathering of sulfur-containing minerals (see p. 77, fig. 5-8)

  10. Nitrogen Fixation Soil air contains ~ 78% N2 gas, however . . . Plants cannot use Nitrogen in the N2 form

  11. Nitrogen Fixation Soil microbes can absorb N2 gas, converting it to ammonia that plants can use • This process is called Nitrogen Fixation • Rhizobium bacteria enter symbiotic relationship with legume plants

  12. Nitrification and Denitrification Nitrogen fixed by microbes is immobilized in the bodies of microbes or their host plant This organic nitrogen is mineralized to ammonium ions (NH4+) when the microbe or plant dies

  13. Nitrification and Denitrification While some NH4+ may be absorbed by plants, most ammonium is oxidized to Nitrites (NO2-) then to Nitrates (NO3-) first by: Nitrosomonas then by: Nitrobacter bacteria Nitrates are then taken up by plants or by other microbes This completes the process of Nitrification

  14. Nitrification and Denitrification • Denitrification occurs when nitrates are changed by anaerobic bacteria back to N2 gas • These denitrifying bacteria use nitrates instead of oxygen • Denitrification occurs most rapidly in wet soils (see p. 79 for series of steps)

  15. Mycorrhizae“fungus root” • Fungi that act as extensions of plant roots Form symbiotic relationships: fungi get sugars produced by plants (~15%) plants get Phosphorus additionally: water, zinc, copper, longer life, protection from disease/toxins

  16. Mycorrhizae • Two types: • Ectomycorrhizae • Outer cells • Tree seedlings • Endomycorrhizae • Inner cells of most plants • VAMs (vesicular-arbuscular mycorrhizae)

  17. Microorganisms • Additional functions: • Break down chemicals (bioremediation) • Certain anaerobics produce methane while others oxidize the methane to carbon dioxide • Organic acids produced dissolve minerals • Rhizospere microbes may produce plant hormones and vitamins, improve nutrient uptake or suppress root disease

  18. Promoting Soil Organisms • Enhance plant functions by: Inoculation puposely infecting soil with useful organisms • Mycorrhizae inoculants improve transplants • Legumes inoculated with Rhizobium for nitrogen fixation

  19. Soil Conditions • Healthy soils provide good microorganism growth • Fresh O.M. is needed • Good aeration for most • Adequate moisture/temperature • Neutral pH • Nutrient requirements (note Nitrogen need)

  20. Soil Animals • Nematodes - Some harmful, some beneficial - Pierce with stylet-type mouthparts - May lead to subsequent pathogen infection • Arthropods – mix soil • Earthworms – benefit most agriculture • Mammals – mix soil

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