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 CO 2, 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) and Heterotrophs (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% N 2 gas, however... Plants cannot use Nitrogen in the N 2 form

Nitrogen Fixation Soil microbes can absorb N 2 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 (NH 4 + ) when the microbe or plant dies

Nitrification and Denitrification While some NH 4 + may be absorbed by plants, most ammonium is oxidized to Nitrites (NO 2 - ) then to Nitrates (NO 3 - ) 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 N 2 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: Inoculation puposely 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