Beneath every healthy ecosystem lies a hidden world of microorganisms that drive some of the most important processes in nature. Soil Microorganisms and Their Functions explores the roles of bacteria, fungi, actinomycetes, algae, and other microbes in nutrient cycling, organic matter decomposition, soil fertility, and ecosystem productivity. Understanding these microscopic communities is essential for interpreting soil health, plant growth, and biogeochemical cycles, key themes in UGC-NET/JRF, SLET, ARS, GATE, and other competitive examinations.
Use this curated MCQ bank to test your conceptual understanding, identify weak areas, and sharpen your exam readiness.
Syllabus Outline
- Diversity, taxonomy, ecology, and physiology of soil microbes.
- Role of bacteria, fungi, archaea, protozoa, and viruses in soil.
- Microbial roles in nutrient cycling and organic matter decomposition.
- Soil structure formation, plant-microbe interactions, and bioremediation.
- Soil microorganisms and soil health, fertility, and ecosystem sustainability.
- Soil microbial isolation, enumeration and characterisation.
Quick Study Guide
Soil microbiology explores the biochemical activities of bacteria, fungi, actinomycetes, and protozoa that drive global nutrient cycles, govern soil fertility, and maintain ecosystem functionality.
- Biological Nitrogen Fixation and Transformations: Nitrogen is a major limiting nutrient, and its cycle is heavily mediated by specific bacteria. Symbiotic nitrogen fixers (Rhizobium) form root nodules in legumes to convert atmospheric N2 into ammonia, while free-living fixers (Azotobacter) do so independently. Nitrification is a two-step aerobic process where Nitrosomonas oxidises ammonia to nitrite (NO2–), and Nitrobacter oxidises nitrite to nitrate (NO3–). Conversely, anaerobic denitrification (Pseudomonas) reduces nitrate back into gaseous N2, completing the loop.
- Mycorrhizal Symbiosis and Phosphorus Mobilisation: Phosphorus often remains locked in insoluble organic or mineral forms. Phosphorus-solubilising bacteria secrete organic acids (e.g. citric or oxalic acid) that chelate bound cations, releasing plant-available phosphate. Additionally, Mycorrhizal fungi form symbiotic networks with plant roots. Ectomycorrhizae wrap around root tips, while Endomycorrhizae (Arbuscular Mycorrhizal Fungi) penetrate cortical cells, drastically expanding the root surface area to scavenge immobile phosphorus.
- Carbon Mineralisation and Humification: Heterotrophic soil microbes drive the carbon cycle by decomposing complex organic residues. Microbes secrete extracellular enzymes to break down tough structural polymers like cellulose and lignin. This process results in mineralisation (converting organic carbon into CO2) and humification (condensing residual organic compounds into highly stable, chemically complex humus that enhances soil moisture and cation retention).
- Actinomycetes and Secondary Metabolites: Actinomycetes are filamentous, branching bacteria that bridge the structural gap between fungi and bacteria. They excel at degrading highly resistant biopolymers such as chitin and cellulose in arid or alkaline soils. Actinomycetes synthesise secondary metabolites, including clinically vital antibiotics, and produce geosmin, the volatile organic compound responsible for the distinct earthy aroma of wet soil.
- Rhizosphere Dynamics and Soil Aggregation: The rhizosphere is the narrow zone of soil directly influenced by root exudates (sugars, amino acids, and phenolics). This nutrient-rich zone fuels a massive microbial population boom known as the rhizosphere effect. Plant Growth-Promoting Rhizobacteria synthesise auxins to stimulate root elongation. Concurrently, fungal hyphae and bacterial exopolysaccharides act as biological glues, binding microparticles into stable soil aggregates that prevent erosion and improve aeration.
Test Your Knowledge
This quiz contains 25 concept-based MCQs on “Soil Microorganisms and their Functions“. Each question has a single correct/most appropriate answer.
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1. Arbuscular mycorrhizal fungi form symbiotic associations with plant roots to enhance:
a) Soil compaction
b) Water-holding capacity
c) Nutrient uptake
d) Soil pH
2. Which soil microorganism plays a key role in cellulose and lignin decomposition?
a) Bacteria
b) Fungi
c) Algae
d) Protozoa
3. Conversion of ammonia to nitrate is primarily carried out by:
a) Nitrogen-fixing bacteria
b) Denitrifying bacteria
c) Ammonifying bacteria
d) Nitrifying bacteria
4. Mycorrhizal fungi assist in plant nutrient uptake by:
a) Breaking down complex organic molecules
b) Fixing atmospheric nitrogen
c) Absorbing and transporting minerals to plant roots
d) Increasing soil acidity
5. Which soil microorganism is responsible for nitrate to nitrogen gas conversion, thereby completing the nitrogen cycle?
a) Nitrifying bacteria
b) Denitrifying bacteria
c) Nitrogen-fixing bacteria
d) Ammonifying bacteria
6. Azotobacter and Azospirillum are examples of soil bacteria known for their ability to:
a) Fix atmospheric nitrogen
b) Decompose cellulose
c) Produce methane gas
d) Synthesise antibiotics
7. Which soil microorganism plays a crucial role in dead plant decomposition?
a) Protozoa
b) Actinomycetes
c) Nematodes
d) Algae
8. Which soil microorganism is responsible for converting atmospheric nitrogen gas into ammonia?
a) Nitrifying bacteria
b) Denitrifying bacteria
c) Nitrogen-fixing bacteria
d) Ammonifying bacteria
9. Nitrogen fixation is inhibited by:
a) Oxygen
b) Carbon dioxide
c) Nitrogen gas
d) Methane
10. Soil microorganisms known as siderophores are involved in the:
a) Fixation of atmospheric nitrogen
b) Decomposition of cellulose
c) Uptake of iron by plants
d) Production of methane gas
11. Mycorrhizal fungi improve plant nutrient uptake by:
a) Producing toxins that kill pathogens
b) Increasing soil compaction
c) Extending the root surface area
d) Decreasing soil pH
12. Which soil microorganism is involved in soil aggregate formation?
a) Bacteria
b) Fungi
c) Algae
d) Viruses
13. Which of the following soil microorganisms is known for its ability to fix atmospheric nitrogen in symbiotic association with leguminous plants?
a) Azotobacter
b) Rhizobium
c) Nitrosomonas
d) Pseudomonas
14. Which of the following is a gram-positive soil bacterium known for producing a toxin or crystal protein (Cry), lethal to caterpillars?
a) Bacillus thuringiensis
b) Lactobacillus
c) Escherichia coli
d) Rhizobium
15. What will be the number of bacterial cells after the successful completion of the 5th generation, if the starting number of cells is 100 in the soil?
a) 32
b) 320
c) 3200
d) 500
16. Which one of the following soil microorganisms is an example of an autotroph?
a) Cyanobacteria
b) Agrobacterium
c) Rhizobium
d) Saccharomyces
17. The relationship of nitrogen-fixing bacteria with root legumes is an example of:
a) Mutualism
b) Protocooperation
c) Commensalism
d) Neutralism
18. Which organism can transform NH4+ into NO2–?
a) Nitrosomonas.
b) Nitrobacter.
c) Pseudomonas.
d) Rhizobium.
19. What is the possible path of nitrogen movement in the environmental compartment?
a) Soil-animals-plants-air-soil
b) Soil-air-plants-air-animals-soil
c) Air-soil-plants-animals-soil-air
d) Air-animals-soil-plants-air
20. What is secreted by the legume root and taken up by the rhizobial symbionts to activate the expression of plasmid-encoded nod (nodulation) genes?
a) Glycolipids
b) Flavonoids
c) Chitolipooligosaccharides
d) Leghemoglobin
21. Which of the following is an example of sulfate-reducing bacteria?
a) Desulfovibrio.
b) Pseudomonas.
c) Nitrobacter.
d) Thiobacillus.
22. What is the main function of VAM?
a) Production of plant growth hormones
b) Production of poison
c) Production of proteins
d) Production of lipids
23. Which of the following is an example of an iron-oxidising archaeon?
a) Ferroplasma acidarmanus
b) Clostridium
c) Bacillus
d) Anthrax
24. Which category do the majority of soil extremophiles belong to?
a) Eukarya
b) Archaea
c) Bacteria
d) Viruses
25. Which of the following forms of nitrogen can be used by microorganisms in place of molecular oxygen?
a) NH4+
b) NO3–
c) N2
d) Ammonia
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Next: Radioactive, Thermal and Marine Pollution
References
- De, Anil Kumar and De, Arnab Kumar (2024). Environmental Chemistry, New Age International, 11th Edition.
- Odum, Eugene P., and Barrett, Gary W. (2004). Fundamentals of Ecology, Thomson Brooks/Cole, 5th Edition.
- Singh, J.S., Gupta, S.R., Singh, S.P. & Singh, R. (2026). Ecology, Environmental Science and Conservation, S Chand Publishing, 2nd Edition.
- Erach Bharucha (2017). Environmental Studies, Universities Press, 4th Edition.
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