NRES 201 Lecture 32 Outline of Last Lecture I Earthworms II Ants III Termites IV Nematodes V Protozoa Outline of Current Lecture VI Plant Roots VII Algae VIII Fungi IX Bacteria and other soil microbes X Factors affecting microbial growth Current Lecture Plant Roots Represent an important group of soil organisms Typically occupy about 1 of the soil volume Supply C and energy for use by other soil organisms Effects on soil o Physical penetration Roots follow existing cracks and channels Their growth enlarges pores o Moisture uptake Improves soil aggregation by stabilizing organic mineral bonds and promoting soil shrinkage and cracking o Organic C input Stimulates microbial growth and activity Promotes humus formation mainly from belowground biomass which represents 15 70 of total plant biomass Algae Occur mainly near the soil surface because they need light for photosynthesis Some species can function in the dark as heterotrophs Many species are flagellated and motile Cultured for bio energy production Fungi Tremendous diversity with millions of soil fungal species These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Aerobic heterotrophs that require O2 and organic C Fungi can be huge they can be micro or macroscopic Fungal characteristics o Molds o Mushroom Fungi Fungal activities o Residue decomposition vital for maintaining soil fertility and humus formation o Fungi are more efficient than bacteria for utilizing complex organic substrates o Nutrient recycling o Fungal hyphae may improve soil aggregation o Bio control Many fungi are highly beneficial to humankind Some produce toxins that reduce competition from bacteria or other fungi A few species trap nematodes o Pathogenic behavior some fungi can cause plant disease by producing chemicals that allow invasion of plant tissues Mycorrhizae o A term meaning fungus root o Refers to the symbiotic association between a fungus and ant root The fungus extends the root system higher efficiency for uptake of water and nutrients especially P o More important in natural ecosystems than in agricultural soils Bacteria Their presence in soil has been recognized since the 19th century but their vast diversity or population since less than 0 1 can be cultured Molecular techniques have revealed how little we know about soil bacteria Extremely rapid reproduction with a few hours to a few days per generation Highly resistant resting stages aid dispersal by wind water or animal digestive tracts Role in decomposition o Most soil bacteria are heterotrophic organic substrates supply C and energy o Like fungi they live by decomposing organic matter o Bacteria dominate decomposition of animal wastes plant residues high in sugar or protein or when O2 is depleted Bacteria bring about many oxidation and reduction reactions that impact plant nutrition and environmental quality They play a crucial role in N2 fixation Factors Affecting Microbial Growth Organic Inputs cause an immediate increase in microbial populations and activity o Bacteria stimulated by simple substrates such as starch or sugars o Fungi and actinomycetes favored by more complex substrates such as cellulose o Placement matters too Fungi dominate when residues left on the soil surface and the bacteria are more active when decay occurs within the soil Soil moisture and aeration o Microbes are inactive in dry soils and spring to life with wetting o 60 water filled pore space ideal for aerobic microbes o Water logging optimal for anaerobic microbes Some activity in anaerobic micro sites at lower soil moisture contents Soil pH and exchangeable calcium o Microbial activity and diversity maximized by near neutral pH values and high levels of exchangeable Ca o Fungi predominate in acidic soils low pH and especially in forest soils
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