Denitrification converts nitrate to
Answer: A
Denitrifying bacteria use nitrate in place of oxygen as an electron acceptor and reduce it through nitrite and nitrous oxide to dinitrogen gas, which escapes the soil entirely. It is the main route by which fertiliser nitrogen is lost from waterlogged fields.
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Denitrification is favoured by soil that is
Answer: B
Denitrifiers only switch to nitrate as an electron acceptor when oxygen runs short, so ponded, compacted or saturated soils drive the process. Good aeration keeps them respiring oxygen instead, which is why drainage conserves nitrogen fertiliser.
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Which genus is a common denitrifier in agricultural soils?
Answer: C
Many Pseudomonas species respire nitrate when oxygen is scarce, making them prominent denitrifiers. Nitrosomonas oxidises ammonium in the opposite direction, Rhizobium fixes nitrogen and Streptomyces is chiefly a decomposer and antibiotic producer.
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The release of ammonia from nitrogenous organic matter by microorganisms is called
Answer: D
Ammonification is the mineralisation step in which proteins, nucleic acids and urea are broken down and their nitrogen released as ammonium. Nitrification then oxidises that ammonium, denitrification returns nitrate to the air, and fixation brings new nitrogen in from the atmosphere.
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Immobilisation of soil nitrogen means that nitrogen is
Answer: A
When microbes decompose material poor in nitrogen they draw mineral nitrogen from the soil into their own biomass, locking it away until they die. It is the reverse of mineralisation, and the balance between the two decides whether a crop is fed or starved after residues are ploughed in.
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Incorporating straw with a very wide carbon to nitrogen ratio into soil usually causes
Answer: B
Decomposers need roughly one part nitrogen to every twenty-five of carbon, so material far wider than that forces them to scavenge mineral nitrogen from the soil, immobilising it. The nitrogen returns as those microbes die, but the crop may go short in the meantime.
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The most abundant organic compound entering soil from plant residues is
Answer: C
Cellulose makes up the bulk of plant cell walls and therefore the bulk of residue carbon. Lignin is present in smaller quantity but decomposes far more slowly, chitin comes mainly from fungi and arthropods, and starch is a storage compound rather than a structural one.
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Which component of plant residue is the most resistant to microbial decomposition?
Answer: D
Lignin is a complex, irregular aromatic polymer with no single repeating bond for enzymes to attack, so only a limited set of organisms, chiefly white-rot fungi, degrade it appreciably. Sugars and proteins disappear within days, and cellulose within weeks to months.
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Humus in soil is best described as
Answer: A
Humus is what remains after the readily decomposable fractions have gone: dark, colloidal and turning over on a timescale of decades or longer. It holds water and cations far out of proportion to its mass, which is why organic matter matters so much to soil fertility.
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Which organisms are chiefly responsible for degrading lignin in soil?
Answer: B
White-rot fungi secrete peroxidases and laccases able to attack the aromatic rings of lignin, a capability very few organisms possess. The bacteria listed work on nitrogen compounds, and algae photosynthesise rather than decompose.
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