Soil Biology and Ecology
16 | Unit 2.3
f ) Root exudates
i. Amounts
• 20–50% more C enters the soil from exudates, sloughed cells, and root hairs than
is present as fibrous roots at end of growing season = substantial contribution to
SOM
• Amount of exudates increased by:
- wetting, after a drying spell
- physical or chemical injury (i.e., mowing, grazing of perennial grass cover crop)
- abrasion, phytotoxic residues, osmotic stress
ii. Types
• Carbohydrates and amino acids: Most-researched compounds
- 10 sugars, glucose and fructose most common
- 25 amino acids
• Also organic acids, fatty acids, sterols, enzymes, volatile compounds, and growth
factors
• Difficult to separate plant and microbe sources
iii. Exudates released from meristem zone
• Nematodes and zoospores congregate there
iv. Foliar sprays may move into roots (depends on molecular weight)
• Herbicides, antibiotics may also move into roots
• Streptomycin moved from Coleus leaves to roots in 24 hrs
• Bacteria suppressed by the streptomycin
g) Variations in root exudates
i. r effect increases with age
ii. r effect decreases with senescence of plants
iii. Annual crop plants have greater r effect than trees
iv. Legume r effect stronger than non-legume
v. r effect may be strongest at flowering
vi. Stronger in sandy soils than in heavier soils
vii. Highest r effect in dune and desert soils
h) Management effects
i. Synthetic fertilizers
• Sometimes no effect
• Sometimes increase r:s indirectly through stimulation of plant growth
ii. Organic manures
• Same indirect positive effect on r:s
• Also may decrease ratio since edaphic (s) microbes are also stimulated by organic
matter input
• After 4 weeks of decomposition, r:s generally increases
- Soil organisms
a) Bacteria
i. Most responsive to plant exudates
ii. 2 to 20 fold increase in bacterial populations in r vs. s
iii. Pseudomonas most consistently abundant in rhizosphere
iv. Also Agrobacterium (biocontrol agent) and Achromobacter
Students’ Lecture Outline