Soil Biology and Ecology
18 | Unit 2.3
- Rotations
a) Monocultures and clean cultivation
i. Create little habitat for soil organisms, leading to less abundant and diverse soil
ecosystems
ii. Consistent plant hosts may serve to develop populations of pathogenic organisms,
causing pest problems, crop losses, or need for pesticide use.
b) Complex rotations
i. Results in greater variety of food sources (roots, root exudates, and residues)
ii. Increases diversity of soil organisms, leading to increased competition for resources,
as well as predation of pathogens and pests.
iii. Interrupts plant-host cycle
c) Multiculture
i. growing more than one crop in one field
ii. More closely mimics natural ecosystem
iii. Likely to support even greater diversity of soil organisms, especially invertebrates
iv. Interrupts plant-host cycle
- Biocides (insecticides, herbicides, fungicides)
a) Effects vary depending on
i. Type of chemical
ii. Species of soil organism in question
iii. Concentration and other exposure factors
b) High levels of pesticide use generally reduce food web complexity
i. Methyl bromide and other fumigants are extreme examples, resulting in temporary
soil sterilization
ii. Eliminate most organisms
iii. Some bacteria quickly return
iv. Other organisms only slowly return
c) Biocides and predator-release phenomenon
i. In cases where biocides selectively eliminate predators, lower trophic levels may
become more abundant
ii. Destabilizing effect on food webs
• Overgrazing on food sources resulting in depletion of food sources
• Population explosion, followed by crash, resulting in...
• Immobilization of nutrients, followed by rapid mineralization at a rate that is not
necessarily compatible with crop needs. May result in leeching of water-soluble
nutrients.
d) Earthworms
i. Most strongly effected (negatively) by fungicides and fumigants
ii. Herbicides
• Don’t seem to be directly toxic
• Indirect negative effect through elimination of vegetation
- Food web structures
a) Fungi/bacteria ratio
b) Dominant microbe influences other trophic levels
Students’ Lecture Outline