Rhizobium fixes atmospheric nitrogen in the root nodules of

The correct answer is B, humus. Humus is the dark, formless material left in soil after decomposers have broken down dead leaves, roots and animal remains; it resists further microbial attack and decays only very slowly, which makes it the long term store of carbon and nutrients in soil and a key step in every biogeochemical cycle. Option A is wrong because chitin is a structural polysaccharide found in the exoskeleton of insects and in fungal walls; it is tough but is a definite compound of living bodies, not an amorphous soil residue. Option C is wrong because carotenoids are orange and yellow plant pigments that help in photosynthesis. Option D is wrong because colloid describes a state of dispersion, not a substance, and humus itself behaves as a soil colloid.

The correct answer is C, the phosphorus cycle. Phosphorus has no stable gaseous compound in nature, so its reservoir is rock, chiefly the mineral apatite; it enters the soil only through slow weathering, passes through plants and animals, and returns to sediments, which makes it a purely sedimentary cycle and the slowest of the common cycles. Options A, B and D are wrong because carbon, nitrogen and oxygen all have large atmospheric reservoirs and move as gases, carbon dioxide, nitrogen and oxygen respectively, which is why they are called gaseous cycles. Sulphur is a mixed case, mainly sedimentary but with sulphur dioxide and hydrogen sulphide in the air. Remember the pairing: phosphorus and calcium sedimentary, carbon and nitrogen gaseous. Each cycle also has a large slow reservoir pool and a small fast exchange pool.

The correct answer is B, Nitrosomonas. Nitrification happens in two stages, and the first is the oxidation of ammonia to nitrite by chemoautotrophic bacteria of the genera Nitrosomonas and Nitrococcus. Option A is wrong because Nitrobacter performs the second stage, oxidising nitrite further to nitrate, the form in which plants mainly absorb nitrogen; questions often swap the two, so the order Nitrosomonas then Nitrobacter is worth memorising. Option C is wrong because Pseudomonas is a denitrifying bacterium that reduces nitrate back to nitrogen gas and so works in the opposite direction. Option D is wrong because Rhizobium fixes atmospheric nitrogen into ammonia inside root nodules, a step that comes before nitrification begins. The order worth memorising is ammonia to nitrite by Nitrosomonas, then nitrite to nitrate by Nitrobacter, and both of these bacteria are chemoautotrophs that take their energy from the oxidation itself.

The correct answer is C, Pseudomonas and Thiobacillus. Denitrification is the reduction of nitrate to nitrogen gas, which escapes to the atmosphere and completes the nitrogen cycle; it is carried out by these bacteria in waterlogged soils where oxygen is scarce, and it is the reason farmers lose nitrogen from flooded fields. Option A is wrong because Nitrosomonas and Nitrococcus oxidise ammonia to nitrite, the first step of nitrification. Option B is wrong because Azotobacter and Clostridium are free living nitrogen fixers, which bring nitrogen in rather than send it out. Option D is wrong because Nostoc and Anabaena are cyanobacteria and also fix nitrogen, Anabaena notably in association with the fern Azolla in rice fields. Denitrification is the only step of the cycle that moves nitrogen out of the soil and back to the air.

The correct answer is B, 78 per cent. Dry air is about seventy-eight per cent nitrogen by volume, which makes the atmosphere the great reservoir of the nitrogen cycle, yet the triple bond of the nitrogen molecule is so strong that plants and animals cannot use the gas directly and depend on fixation. Option A is wrong because twenty-one per cent is the share of oxygen, the second most abundant gas. Option C is wrong because about four hundredths of one per cent is the share of carbon dioxide, small in quantity but decisive for photosynthesis and for the greenhouse effect. Option D is wrong because no single gas forms ninety-three per cent of air; argon, the commonest of the inert gases in air, is well under one per cent.