Teaching Organic Farming and Gardening

(Michael S) #1

Soil Biology and Ecology


6 | Unit 2.3
Instructor’s Lecture Outline


d) See appendix 1, Major Organic Components of Typical Decomposer Food Sources,

for a comparison of the components of some typical decomposer food sources


e) Physical factors influence decomposition


f) Limiting factors


g) Plant secondary compounds may inhibit decomposition (polyphenols, tannins)



  1. Nitrogen cycle


a) Proteins —> amino acids —> ammonium —> nitrate


b) Ammonification aerobic or anaerobic


c) Nitrification aerobic


d) If C:N < 20–30:1 = net mineralization. If C:N > 20–30:1 = net immobilization



  1. Carbon and nitrogen mineralization exercise (handout)


E. Soil Food Webs



  1. Soil food web ecology

  2. Heterotrophic vs. autotrophic food webs


a) Autotrophic food webs —> begin with C fixation by plants


b) Heterotrophic food webs —> release nutrients required by all plants


c) Energy loss = 80–90% at each step in the food chain


d) Food web structure


e) Ways that soil animals interact with soil microorganisms


f) Unique food web for each ecosystem, determined by:


F. Soil Biota



  1. Characteristics

  2. Habitats

  3. Functional classification


a) Microorganisms


b) Microfauna


c) Mesofauna


d) Macrofauna


e) Megafauna


G. Rhizosphere Ecology



  1. Definitions


a) Rhizosphere (R) = the narrow zone of soil subject to the influence of living roots,

as manifested by the leakage or exudation of substances that promote or inhibit
microbial activity


b) Rhizoplane (r)= the actual root surface, which provides a highly favorable nutrient

base for many species of bacteria and fungi


c) Edaphosphere (S) = soil beyond root influence


d) Rhizosphere effect = soil microorganisms and fauna stimulated


i. —> R/S ratio generally increases


e) Rhizosphere succession = the sequence of changes in the area surrounding a growing
root

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