Decomposition of a residue will release mineral nitrogen rather than immobilise it when the residue's carbon to nitrogen ratio is
Answer: A
Below roughly twenty to twenty-five to one the material carries more nitrogen than the decomposers need for the carbon it supplies, so the surplus is released as ammonium. Above that they must take nitrogen from the soil, and the crop can go short.
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Which residue would you expect to decompose most rapidly in soil?
Answer: B
Young leguminous growth is high in nitrogen and low in lignin, so it has a narrow carbon to nitrogen ratio and few structural barriers. Straw, sawdust and wood are progressively wider in ratio and richer in lignin, and break down over months to years.
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Mineralisation in soil refers to the conversion of
Answer: C
Mineralisation releases nutrients from organic matter into forms such as ammonium, phosphate and sulphate that plants can take up. The reverse, incorporation into microbial tissue, is immobilisation, and the conversion of nitrate to gas is denitrification.
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Which statement about soil fungi is correct?
Answer: D
Individual hyphae are far larger than bacterial cells, so although fungi are vastly outnumbered they commonly carry the greater share of biomass, particularly in undisturbed and acidic soils. They decompose cellulose readily and are the principal degraders of lignin.
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Nitrogen fixation by free-living aerobic bacteria such as Azotobacter is limited mainly by
Answer: A
Fixing nitrogen is expensive in energy, and a free-living organism must pay for it from soil organic carbon, which is usually scarce. A nodulated Rhizobium avoids the problem because the host supplies sugars directly, which is why symbiotic fixation contributes so much more.
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Which term describes microorganisms that obtain both energy and carbon from organic compounds?
Answer: B
The prefix chemo names the energy source as chemical and hetero names the carbon source as organic, so chemoheterotroph covers the great majority of soil organisms. Nitrifiers by contrast are chemoautotrophs, taking energy from inorganic oxidation and carbon from carbon dioxide.
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Soil respiration is commonly measured as
Answer: C
Trapping and measuring the carbon dioxide released gives an integrated measure of the activity of the whole community, including organisms that will not grow on any medium. It reports activity rather than population size, which is what a plate count attempts.
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Which of these is a nitrogen-fixing bacterium that lives closely associated with the roots of grasses and cereals without forming nodules?
Answer: D
Azospirillum colonises the root surface and outer tissues of grasses in an associative rather than nodulating relationship, benefiting from root exudates while fixing nitrogen. The other three genera oxidise nitrite, reduce sulphate and oxidise sulphur respectively.
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The bulk of nitrogen entering agricultural soils biologically comes from
Answer: A
Symbiosis dominates because the host supplies the large amount of energy fixation demands, letting nodulated legumes add far more per hectare than free-living organisms can. Lightning contributes a small amount, and nitrifiers only transform nitrogen already present.
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A soil sample for microbiological analysis should ideally be
Answer: B
Drying, heating or repeated freezing all change the community before it is measured, and storage lets the population shift. Keeping the sample near field moisture at a low temperature and analysing it quickly gives the closest picture of the soil as it was.
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