Which group of soil microorganisms is generally the most numerous per gram of soil?
Answer: A
Bacteria outnumber every other group by orders of magnitude: a single teaspoon of productive soil holds between a hundred million and a billion of them. Fungi usually exceed bacteria in total BIOMASS because their hyphae are far larger, which is why the question asks about numbers rather than weight.
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The earthy smell of freshly turned soil is produced by
Answer: B
Actinomycetes release geosmin as they die, and it is this compound the human nose detects at very low concentrations after rain or ploughing. The other gases listed do occur in soil, but hydrogen sulphide smells of rotten eggs and neither ammonia nor methane gives the characteristic earthy note.
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Which soil organisms are the principal natural source of antibiotics such as streptomycin?
Answer: C
Streptomyces, a genus of actinomycetes, produces more than fifty distinct antibiotics, streptomycin among them, which help suppress competing organisms in the soil. Rhizobium fixes nitrogen, Nitrosomonas oxidises ammonium and Thiobacillus oxidises sulphur; none of them is an antibiotic source.
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Actinomycetes are best described as
Answer: D
Actinomycetes are prokaryotes, so despite the filamentous growth habit that once had them classed with fungi they are bacteria: they have no nucleus, their filaments are far narrower than fungal hyphae and their cell walls contain peptidoglycan rather than chitin.
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The rhizosphere is best defined as
Answer: A
The rhizosphere is the narrow region of soil whose chemistry and biology are altered by the root, chiefly through exudates of sugars, amino acids and organic acids. The root SURFACE is distinguished as the rhizoplane, and the litter layer and the saturated zone are unrelated to root influence.
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The R:S ratio used in soil microbiology compares
Answer: B
R:S divides the microbial count in the rhizosphere by the count in the surrounding bulk soil, so it measures how strongly the root stimulates microbial growth. It is highest for bacteria, around twenty to one, and lower for fungi and actinomycetes. The other expansions of the letters are inventions.
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The rhizosphere effect is caused mainly by
Answer: C
Roots leak sugars, amino acids, organic acids and sloughed cells, and this carbon supply is what lets microbial populations near the root reach many times the density of bulk soil. Soil in that zone is not measurably warmer, and any antimicrobial exudates would reduce the population rather than raise it.
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The term rhizoplane refers to
Answer: D
The rhizoplane is the root surface itself, along with the soil that clings to it, and it carries an even denser microbial film than the surrounding rhizosphere. Distinguishing the two matters when a method requires washing the root, which removes rhizosphere soil but leaves the rhizoplane community.
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Atmospheric nitrogen is converted to ammonia by the enzyme
Answer: A
Nitrogenase is the enzyme complex that reduces atmospheric dinitrogen to ammonia, and it is found only in prokaryotes. Nitrate reductase works in the opposite direction of the cycle, urease releases ammonia from urea, and catalase merely destroys hydrogen peroxide.
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Which genus forms nitrogen-fixing root nodules on leguminous plants?
Answer: B
Rhizobium infects legume roots and induces nodules in which it fixes nitrogen for the host in exchange for carbon. Azotobacter and Clostridium fix nitrogen too, but they live free in the soil and form no nodules, and Nitrobacter oxidises nitrite rather than fixing anything.
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