Microbiology of Soils Questions and Answers

71.

Blue-green algae used as a biofertiliser in rice fields contribute mainly by

A.Killing the larvae of insect pests in the water
B.Lowering the temperature of the flooded field
C.Fixing atmospheric nitrogen in the flooded field
D.Solubilising rock phosphate applied to the field

Answer: C

Correct answer: Fixing atmospheric nitrogen in the flooded field

The cyanobacteria used in rice paddies - Anabaena, Nostoc and their relatives, often in symbiosis with the water fern Azolla - fix atmospheric nitrogen in their heterocysts and release it to the water and soil as they die and decompose. A flooded field suits them because they are photosynthetic and need standing water, and the contribution can amount to several tens of kilograms of nitrogen per hectare per season.

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72.

The specialised cell in which many filamentous cyanobacteria fix nitrogen is the

A.Akinete
B.Arbuscule
C.Bacteroid
D.Heterocyst

Answer: D

A heterocyst has a thickened wall and lacks the oxygen-producing photosystem, giving nitrogenase the anaerobic interior it requires. An akinete is a resting spore of the same organisms, an arbuscule belongs to mycorrhizas and a bacteroid is the fixing form of Rhizobium inside a nodule.

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73.

Inside a legume nodule, the Rhizobium cells that actually fix nitrogen are called

A.Bacteroids
B.Heterocysts
C.Endospores
D.Zoospores

Answer: A

On entering the host cell the bacteria enlarge, change shape and lose the ability to divide, becoming bacteroids devoted to fixation. Heterocysts serve the same purpose in cyanobacteria, while endospores and zoospores are survival and dispersal stages.

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74.

Green manuring improves soil nitrogen most when the crop ploughed in is

A.A grass
B.A legume
C.A cereal
D.A root crop

Answer: B

A legume carries nitrogen fixed from the atmosphere by its root nodules, so incorporating it adds nitrogen the soil did not previously hold. Non-legumes merely recycle nitrogen already present, and their wide carbon to nitrogen ratio may even immobilise more of it for a time.

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75.

Legume seed is often inoculated before sowing in order to

A.Speed up germination by softening the seed coat directly
B.Add a starter dose of phosphorus to the seedbed
C.Ensure an effective Rhizobium strain is present in the soil
D.Protect the sown seed against birds and other seed predators

Answer: C

A soil that has not carried that legume recently may hold no compatible strain, or only ineffective ones, so inoculation places the right partner where the emerging root will meet it. It supplies no nutrient itself and has no direct effect on germination.

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76.

Repeatedly growing the same crop without rotation tends to

A.Increase the diversity of the soil organism community
B.Eliminate the soil fungal population altogether
C.Raise the soil organic matter content steadily
D.Build up populations of that crop's soilborne pathogens

Answer: D

A constant host lets its specialised pathogens accumulate season after season, which is the main reason rotation is practised. Rotation also varies the residue chemistry reaching the soil and so supports a broader community, the opposite of what continuous cropping does.

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77.

Deep ploughing of a soil generally causes organic matter to

A.Decline, because aggregates are broken and decomposition speeds up
B.Rise, because crop residues are buried where they decay slowly
C.Stay exactly constant, since ploughing only moves soil about
D.Convert directly into humus without passing through microbial biomass

Answer: A

Cultivation shatters aggregates and exposes the organic matter they protected, so decomposers reach carbon that was previously physically inaccessible and respiration rises. Burial slows losses from the surface but does not offset the overall decline.

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78.

Adding lime to an acidic soil generally

A.Converts existing humus back into plant cellulose
B.Increases bacterial activity, including nitrification
C.Reduces microbial activity of every kind in the soil
D.Has no measurable effect on the soil organisms present

Answer: B

Raising pH towards neutral suits bacteria and especially the nitrifiers, which prefer conditions above about pH 7, so mineralisation and nitrification both speed up. Fungi lose the competitive advantage that acidity gave them.

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79.

Which practice would most directly increase the soil microbial biomass?

A.Deep ploughing every season so as to aerate the whole profile
B.Keeping the soil bare and dry between successive crops
C.Regularly adding organic matter such as compost or residues
D.Applying a broad-spectrum fumigant before each planting

Answer: C

Microbial biomass is limited above all by carbon supply, so returning organic material feeds it directly. Fumigation kills the community outright, and cultivation and bare fallow both accelerate the loss of the organic matter it depends on.

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80.

The carbon to nitrogen ratio of soil microbial biomass is roughly

A.Eighty to one
B.One to eight
C.Four hundred to one
D.Eight to one

Answer: D

Microbial tissue is protein rich and narrow in carbon relative to nitrogen, around eight to one, which is far narrower than the plant residues they consume. That gap is exactly why decomposing wide-ratio material forces them to draw mineral nitrogen out of the soil.

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