Soils 205- general soils

Lecture 24- Soil organisms

I.  simple classification of soil organisms

    a.  Animals

        1.  Macrofauna

                > 2 mm body width (earthworms, moles, milllipedes)

        2.  Mesofauna

                0.2-2 mm (mites)

        3.  Microfauna

            < 0.2 mm (nematodes)

        i. Example macrofauna- earthworms

-feed on detritus, soil organic matter (SOM), and the microbes living on these tissues

-burrowing creates macropores- enhances drainage and aeration

-casts increase aggregate stability, provide mesopores for water retention

-incorporate organic material into soil

-organic matter is ground within the gut; this decreases particle size and creates more surface area for microorganisms to attack

-serve as the major means of incorporation in no-till and in many wildland systems

    ii.  Mesofauna- mites

-second most diverse animal group

-may feed on detritus (decaying material), plant material (phytophagous), fungus (fungivorous), or insects (insectivorous)

    iii.  Microfauna- nematodes

-unsegmented roundworms

-feed on fungi, bacteria, algae, protozoa and insect larvae

-grazing affects bacteria and fungi populations

-some are plant parasites

b.  soil flora

    1.  plant roots

    -occupy ~1% of the soil volume, but account for 25-35% of the respiration

    -supply much of the C and energy needed by organisms

        grasslands- major source of C is added to the soil through root death

        forests- ~ 50% of the total biomass production

-Rhizosphere- zone around the root about 2mm thick.  Microorganisms numbers are about 2-10 x's greater than in bulk soil

    2.  bacteria

    -typically 0.5 to 5 um

    -very diverse group- 1 gram of soil contains ~ 20,000 species

    -breakdown organic materials (wide range of enzymatic cababilities)

    -function in N fixation

    3.  fungi

    -tens of thousands of species; may be as many as 1 million in soil

    -have filaments called hyphae

    -aerobic heterotrophs that decompose a wide variety of organic substances (cellulose, lignin, etc)

    -very important in forest soils

    -form mycorrhizae "fungus root"- symbiotic associations between certain fungi and roots of plants.  The  association greatly enhances the ability of a plant to take up P, may also improve water uptake

    -used as "biocontrol" agents

    4.  algae

    -autotrophs- have chlorophyll and can photosynthesize; need light so they are generally concentrated near the soil surface

    -provide organic matter to soils

    -enter into symbiotic relationships with fungi (lichens)

    5.  Atinomycetes

    -fungi-like bacteria

    -second most numerous after bacteria

    -important in decomposition of organic matter, even very resistant compounds

    -produce many antibiotic compounds- actinomycin, streptomycin

    -produce "giosmins" which give the earthy smell associated with soils

2.  Conditions affecting microbial growth

-plant residue and soil organic matter- serves as energy source for the majority of microbes

-most organisms are aerobic- use O₂ as electron acceptor in metabolism

-moisture and temperature