NRES 201 Lecture 32 Outline of Last Lecture I. EarthwormsII. AntsIII. TermitesIV. NematodesV. ProtozoaOutline of Current Lecture VI. Plant RootsVII. AlgaeVIII. FungiIX. Bacteria and other soil microbesX. Factors affecting microbial growthCurrent LecturePlant 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 poreso Moisture uptake: Improves soil aggregation by stabilizing organic mineral bonds andpromoting soil shrinkage and crackingo Organic C input: Stimulates microbial growth and activity Promotes humus formation - mainly from belowground biomass, whichrepresents 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 productionFungi -- Tremendous diversity with millions of soil fungal speciesThese 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 Moldso 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 substrateso 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 otherfungi A few species trap nematodeso Pathogenic behavior - some fungi can cause plant disease by producing chemicalsthat allow invasion of plant tissues - Mycorrhizaeo 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 waterand nutrients - especially P o More important in natural ecosystems than in agricultural soilsBacteria - - Their presence in soil has been recognized since the 19th century but their vast diversityor 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 energyo Like fungi, they live by decomposing organic mattero Bacteria dominate decomposition of animal wastes, plant residues high in sugaror protein, or when O2 is depleted- Bacteria bring about many oxidation and reduction reactions that impact plant nutritionand environmental quality. - They play a crucial role in N2 fixationFactors Affecting Microbial Growth:- Organic Inputs cause an immediate increase in microbial populations and activity. o Bacteria stimulated by simple substrates such as starch or sugarso Fungi and actinomycetes favored by more complex substrates such as cellulose.o Placement matters too - Fungi dominate when residues left on the soil surfaceand 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 wettingo 60% water - filled pore space ideal for aerobic microbeso Water-logging optimal for anaerobic microbes Some activity in anaerobic micro-sites at lower soil moisture contents- Soil pH and exchangeable calciumo Microbial activity and diversity maximized by near neutral pH values and highlevels of exchangeable Cao Fungi predominate in acidic soils (low pH) and especially in forest
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