Role of micro-organisms
Respiration, fermentation, decomposition of organic matter, and soil flora and fauna.
Microbes at work
Respiration releases energy from food.
- Aerobic respiration uses oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. It yields much more ATP than anaerobic respiration.
- Anaerobic respiration occurs without oxygen and gives less energy.
Fermentation is the anaerobic breakdown of sugars into products such as alcohol or acids.
- Yeast (Saccharomyces): glucose → ethanol + CO₂ (bread, alcohol).
- Lactic acid bacteria (Lactobacillus): lactose → lactic acid, which sets milk into curd.
- Silage making for cattle is also a lactic acid fermentation.
Decomposition of organic matter: bacteria, fungi and actinomycetes break crop residues, dung and dead organisms into simple compounds and humus. The speed depends on moisture, aeration, temperature and the C:N ratio: materials with a wide C:N ratio (straw) decompose slowly and may temporarily lock up soil nitrogen; narrow C:N materials (legume leaves) decompose fast. Composting uses this process under controlled conditions. Decomposition releases nutrients (mineralisation) back to plants.
Microbes in the nitrogen cycle:
- Nitrogen fixation: Rhizobium in root nodules of legumes (symbiotic); Azotobacter (free-living, aerobic); Azospirillum (associative, cereals); blue-green algae.
- Ammonification: organic N → ammonia.
- Nitrification: Nitrosomonas (NH₄⁺ → NO₂⁻), then Nitrobacter (NO₂⁻ → NO₃⁻).
- Denitrification: nitrate → N₂ gas in waterlogged soils (loss of N).
Mycorrhizal fungi help roots absorb phosphorus.
Soil flora and fauna: flora includes bacteria (the most numerous), actinomycetes, fungi and algae. Fauna includes protozoa, nematodes, mites, insects and earthworms, which mix soil, make channels and produce nutrient-rich casts; Darwin called them nature's ploughs.
Key points
- Yeast fermentation → ethanol + CO₂; Lactobacillus → lactic acid (curd).
- Rhizobium: symbiotic; Azotobacter: free-living N-fixer.
- Nitrosomonas: NH₄⁺ → NO₂⁻; Nitrobacter: NO₂⁻ → NO₃⁻.
- Wide C:N ratio → slow decomposition.
Exam tips
Learn organism–process pairs; these are frequent one-liners.
Common mistakes
- Swapping Nitrosomonas and Nitrobacter.
- Thinking denitrification adds nitrogen to soil; it removes it.