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 . . .

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

Detrital Food Chain

Breaks down and recycles dead organisms

• detritus – dead organisms or their products

• decomposers – consume organic matter, release CO2, and leave humus

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

Ecology of the Soil

Five ecological roles of interest to soil students

• Autotrophs (producers) andHeterotrophs (consumers)

• Parasites• Predators• Saprophytes• Symbionts

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

Nutrient Cycling

• Nutrients in living bodies or fresh O.M. are said to be immobilized

• Microbes decompose immobilized nutrients through mineralization

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)

Nitrogen Fixation

Soil air contains ~ 78% N2 gas, however . . .

Plants cannot use Nitrogen in the N2 form

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

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

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

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)

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

Mycorrhizae

• Two types:

• Ectomycorrhizae– Outer cells– Tree seedlings

• Endomycorrhizae – Inner cells of most plants– VAMs (vesicular-arbuscular mycorrhizae)

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

Promoting Soil Organisms

• Enhance plant functions by:

Inoculationpuposely infecting soil with useful organisms

- Mycorrhizae inoculants improve transplants

- Legumes inoculated with Rhizobium for nitrogen fixation

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)

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