Biogeochemical Cycles: Carbon, Nitrogen and Water Cycles

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 A, leguminous plants. Rhizobium is a bacterium that lives in a symbiotic relationship with legumes such as gram, pea, bean, groundnut and arhar, forming nodules on their roots in which it converts nitrogen gas into ammonia using the enzyme nitrogenase; this is why legumes are grown in rotation to restore soil nitrogen. Option B is wrong because cereals form no such nodules and depend on soil nitrate or on fertiliser. Option C is wrong because conifers are not nodulated by Rhizobium, though some non-legumes such as Alnus host a different bacterium, Frankia. Option D is wrong because mosses have no true roots and so no root nodules at all; some do associate loosely with cyanobacteria. Inside the nodule the pink pigment leghaemoglobin keeps oxygen away from nitrogenase, the enzyme that does the fixing.

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.

The correct answer is A, nitrogenase. Nitrogenase is the enzyme complex, containing molybdenum and iron, that breaks the triple bond of the nitrogen molecule and joins the atoms with hydrogen to make ammonia; it is destroyed by oxygen, which is why a root nodule keeps leghaemoglobin to mop oxygen up. Option B is wrong because nitrate reductase works later, inside the plant, reducing absorbed nitrate towards ammonia for making amino acids. Option C is wrong because carbonic anhydrase handles carbon dioxide and water in the blood and belongs to the carbon cycle rather than this one. Option D is wrong because catalase breaks hydrogen peroxide into water and oxygen and protects cells from oxidative damage. Nitrogenase needs molybdenum and iron, which is why these two trace elements matter so much to a legume crop.

The correct answer is B, combining atmospheric nitrogen with oxygen to form oxides of nitrogen. The very high temperature of a lightning discharge supplies enough energy to break the triple bond of nitrogen, and the atoms join oxygen to give nitric oxide and then nitrogen dioxide, which dissolve in rain as nitrous and nitric acid and reach the soil as nitrate; this is physical or atmospheric fixation, a small share of the total. Option A is wrong because lightning has no such effect on soil bacteria. Option C is wrong because humus is broken down slowly by microbes, not by lightning. Option D is wrong because phosphate is released by the weathering of rock, which belongs to the sedimentary phosphorus cycle and not to the nitrogen cycle at all.

The correct answer is B, glaciers and polar ice. About ninety-seven per cent of the water on earth is salt water in the oceans, and of the small freshwater remainder the greater part is locked up as ice in glaciers and the polar ice sheets, with most of what is left held as groundwater. Option A is wrong because rivers and lakes together hold only a tiny fraction of fresh water, even though they supply most of what people use. Option C is wrong because the atmosphere holds the least of all, a mere trace, which is why it must turn over many times a year to produce the rainfall it does. Option D is wrong because soil moisture is likewise a very small store, important to plants but minute in total volume.

The correct answer is B, transpiration. Transpiration is the escape of water vapour from a plant, chiefly through the stomata of the leaves, and over a forest or a crop field it returns a large volume of water to the air; together with evaporation from open surfaces it is called evapotranspiration. Option A is wrong because evaporation is the change of liquid water to vapour from a free surface such as a lake, a sea or wet soil, with no living tissue involved. Option C is wrong because condensation is the reverse change, vapour turning to droplets around dust particles to form cloud. Option D is wrong because percolation is the downward seepage of water through soil and rock to the water table. Transpiration and evaporation taken together over vegetated land are called evapotranspiration.

The correct answer is A, about 0.04 per cent. Carbon dioxide is a trace gas, roughly four hundredths of one per cent of dry air by volume, yet it is the source of all the carbon that photosynthesis fixes into food and the chief gas by which human activity is warming the planet. Option B is wrong because four per cent would be a hundred times the real figure; no atmosphere of that composition could support present life comfortably. Option C is wrong because twenty-one per cent is the share of oxygen. Option D is wrong because about nine tenths of one per cent is the share of argon, the commonest inert gas in air. Nitrogen at seventy-eight per cent and oxygen at twenty-one make up almost all of it.

The correct answer is B, nitrate. Roots take up nitrogen chiefly as the nitrate ion and to a smaller extent as ammonium; inside the plant the nitrate is reduced and built into amino acids, proteins and nucleic acids. Option A is wrong because the triple bond of nitrogen gas is too strong for a plant to break, which is the whole reason fixation by bacteria, lightning or industry is needed. Option C is wrong because nitrogen dioxide is a pollutant gas of the air that reaches soil only after dissolving in rain as nitrate. Option D is wrong because urea applied as fertiliser must first be hydrolysed to ammonia by the enzyme urease in the soil and then nitrified before most of it is taken up.