Questions on nutrient cycles: nitrogen, phosphorus and eutrophication

Question 1

Describe the sequence of events by which nitrate leaching from a fertilised field leads to the death of fish in a lake.

Accepted Answer

B1 nitrate ions are very soluble and are not held by the negatively charged clay and humus particles, so rain washes them down through the soil into the water, relieving the shortage of the nutrient that was limiting the algae B1 algae and cyanobacteria at the surface multiply rapidly to form an algal bloom, because they already had light, carbon dioxide and warmth and now have the nutrient too B1 the bloom forms a dense mat that blocks the light, so submerged rooted plants and deeper algae photosynthesise below the rate at which they respire, and die B1 those dead plants, and the bloom itself once it exhausts the nutrient and dies back, are a vast supply of dead organic matter on which saprobiotic bacteria feed, so the bacterial population grows enormously B1 those bacteria respire aerobically and take dissolved oxygen out of the water faster than it can dissolve back in from the air, so the dissolved oxygen concentration falls and fish and invertebrates needing well-oxygenated water suffocate

Question 2

A campaign calls for artificial nitrogen fertiliser to be banned within one kilometre of any river, in order to stop eutrophication. Evaluate this proposal.

Accepted Answer

B1 for: artificial nitrate is available for uptake immediately after spreading, so it is also immediately available to be washed out by rain before a crop takes it up, and restricting its use near a river removes this readily leached source close to the water B1 for: buffer strips of unfertilised, uncultivated land between a field and a watercourse already reduce how much nitrate reaches the water, so restricting fertiliser near rivers follows an already established remedy B1 against: natural fertilisers such as manure and slurry, and treated sewage, also add nitrogen and phosphate to water and would not be covered by a ban on artificial fertiliser, so a major source of enrichment near the river could remain unaffected B1 against: in fresh water phosphate is usually the nutrient in shortest supply and so the one limiting algal growth, and phosphate reaches water mainly attached to eroded soil rather than dissolved in it, so a ban aimed at nitrogen leaves the main route of the limiting nutrient largely open B1 judgement: a ban restricted to artificial nitrogen fertiliser addresses only one source of one nutrient, and would need to be combined with control of manure, sewage and soil erosion to make eutrophication significantly less likely rather than simply moving its source

Question 3

Compare nitrogen fixation with nitrification, referring to what each starts with, what each produces, the organisms responsible and the part oxygen plays.

Accepted Answer

B1 fixation starts with nitrogen gas from the atmosphere, whereas nitrification starts with ammonium already present in the soil, so only fixation brings nitrogen into the ecosystem B1 fixation reduces nitrogen gas to ammonia, which takes up a hydrogen ion to become ammonium, whereas nitrification oxidises ammonium to nitrite and then nitrite to nitrate B1 fixation is carried out by nitrogen-fixing bacteria, some free-living in the soil and some in the root nodules of legumes, whereas nitrification is carried out by nitrifying bacteria, one kind oxidising ammonium and another oxidising nitrite; genus names such as Rhizobium or Nitrosomonas are not required B1 nitrogenase is destroyed by oxygen, so fixation happens where oxygen is kept away and leghaemoglobin mops it up inside a nodule, whereas nitrification requires oxygen because both of its steps are oxidations

Question 4

Explain why a well-drained, ploughed soil holds more nitrate than a compacted, waterlogged one.

Accepted Answer

B1 ploughing and draining keep air in the spaces between the soil particles, so oxygen is available B1 nitrification is carried out by Nitrosomonas and Nitrobacter and both of its steps are oxidations requiring oxygen, so ammonium is oxidised through nitrite to nitrate and nitrate accumulates B1 in a compacted or waterlogged soil the spaces are filled with water and oxygen is absent, so nitrification stops and no further nitrate is produced B1 denitrifying bacteria such as Pseudomonas then respire using nitrate as the final electron acceptor in place of oxygen, converting it to nitrogen gas that leaves the soil, so the nitrate already there is lost as well

Question 5

A 25-hectare field receives ammonium nitrate, NH₄NO₃, at 200 kg per hectare; take its relative formula mass as 80 and the relative atomic mass of nitrogen as 14. Over the following months a stream draining the field carries 8.0 × 10⁷ litres of water, and its nitrogen concentration is on average 5.6 mg per litre higher than upstream of the field. Calculate the mass of nitrogen applied and calculate what percentage of it reached the stream.

Accepted Answer

M1 there are two nitrogen atoms in NH₄NO₃, so nitrogen is 2 × 14 = 28 of the relative formula mass of 80, which is 28 ÷ 80 = 0.35, or 35 per cent by mass A1 nitrogen applied = 200 × 0.35 = 70 kg per hectare, and over 25 hectares that is 1750 kg M1 nitrogen carried by the stream = 5.6 × 8.0 × 10⁷ = 4.48 × 10⁸ mg, which is 448 kg A1 448 ÷ 1750 = 0.256, so about 26 per cent of the nitrogen applied reached the stream

Question 6

Describe how ammonification returns nitrogen to the soil as ammonium, naming the organisms responsible and what they act on.

Accepted Answer

B1 saprobiotic bacteria and fungi carry out ammonification, feeding on the nitrogen-containing compounds in dead organisms, faeces and urine B1 proteins in that dead material are hydrolysed to amino acids, which are then deaminated, releasing ammonium B1 urea is hydrolysed by the enzyme urease, also releasing ammonium into the soil B1 unlike nitrogen fixation, none of the nitrogen ammonification releases has come from the atmosphere; it was already present in the ecosystem, in the bodies and waste of organisms

Question 7

Outline the pathway phosphorus follows through an ecosystem, from the rock in which it starts to its return to the soil.

Accepted Answer

B1 phosphate is released from rock by weathering and dissolves into the soil solution B1 plants take up phosphate ions from the soil solution, largely with the help of mycorrhizal fungi associated with their roots B1 phosphorus then passes along food chains as animals eat plants and other animals B1 decomposers acting on dead organisms, urine and faeces return phosphate to the soil, and at no stage does it pass through a gaseous form

Question 8

Compare a natural fertiliser such as manure with an artificial fertiliser, referring to the chemical form of the nitrogen each supplies, how quickly that nitrogen becomes available to a crop, and how readily it is lost by leaching.

Accepted Answer

B1 a natural fertiliser holds its nitrogen locked in organic compounds such as protein, whereas an artificial fertiliser supplies nitrogen already as an inorganic salt such as nitrate B1 the nitrogen in a natural fertiliser must first be ammonified and then nitrified before a plant can absorb it, so it becomes available slowly over weeks, whereas the nitrate in an artificial fertiliser is available to the crop immediately B1 because artificial nitrate is available immediately it can also be washed out of the soil by rain before the crop has taken it up, whereas the slow release of a natural fertiliser matches uptake more closely and so is lost less readily B1 a natural fertiliser also adds organic matter that improves soil structure, which an artificial fertiliser, being purely inorganic salts, does not

Question 9

Phosphate is usually the nutrient in shortest supply in fresh water, and the phosphorus cycle has no gaseous stage. Suggest why stripping phosphate from effluent at a sewage works reduces algal blooms in the river below it, and suggest why phosphate carried out to sea is not replaced on any useful timescale.

Accepted Answer

B1 the algae are limited by whichever nutrient is in shortest supply, so removing phosphate leaves that shortage in place and the bloom cannot develop even where other nutrients are plentiful B1 sewage effluent is a continuous point source of phosphate, so treating it at the works removes the input rather than treating the symptom, and one works serves a whole town B1 phosphate has no volatile form, so there is no atmospheric route back to the land; the return from sea-bed sediment to rock by uplift and weathering takes millions of years, so on any human timescale the phosphate is lost

Question 10

Explain why nitrogen gas, which makes up about 78 per cent of the air, cannot be used directly by a plant, and explain which ion a plant absorbs instead.

Accepted Answer

B1 the two nitrogen atoms in N₂ are joined by a triple bond that needs a great deal of energy to break, and neither plants nor animals possess an enzyme able to break it B1 nitrogen is needed to build amino acids and hence proteins and enzymes, nucleotides and hence DNA, RNA and ATP, and chlorophyll, so a plant cannot avoid needing a usable source of it B1 a plant absorbs nitrogen mainly as nitrate ions, taken up into the root hairs by active transport, with a smaller amount taken up as ammonium

Question 11

Explain how a mycorrhizal association benefits both the fungus and the plant it is attached to.

Accepted Answer

B1 fungal hyphae are far finer than root hairs and spread through a much larger volume of soil, greatly increasing the surface area available for absorbing water and mineral ions such as phosphate, which moves through soil very slowly B1 the plant therefore takes up phosphate faster with the fungus present than it could using its root hairs alone B1 in return the fungus receives organic compounds that the plant has made by photosynthesis, which the fungus, lacking chlorophyll, cannot make for itself

Question 12

Explain why nitrate is washed out of soil by rain far more readily than ammonium or phosphate are.

Accepted Answer

B1 nitrate ions carry a negative charge, and clay and humus particles in soil also carry a net negative charge, so the two repel and nitrate is not held onto them B1 ammonium ions carry a positive charge, so they are attracted to and held by the negatively charged soil particles instead of moving freely with the water B1 phosphate is much less soluble than nitrate, so it stays largely bound to soil particles rather than dissolving into the water draining through the soil

Question 13

State two roles nitrogen plays in a living organism, and state one visible symptom of nitrogen deficiency in a plant.

Accepted Answer

B1 nitrogen is needed to build amino acids, and hence proteins and enzymes B1 nitrogen is needed to build nucleotides, and hence DNA, RNA and ATP, and to build chlorophyll B1 a nitrogen-deficient plant shows stunted growth and yellowing of its older leaves

Question 14

Name the genus of bacteria that oxidises ammonium to nitrite, and name the genus that oxidises nitrite to nitrate.

Accepted Answer

B1 Nitrosomonas oxidises ammonium to nitrite B1 Nitrobacter oxidises nitrite to nitrate

Question 15

Name the genus of bacteria that fixes nitrogen inside root nodules on the roots of legumes such as peas and beans, and name the protein that protects its enzyme from oxygen inside the nodule.

Accepted Answer

B1 Rhizobium fixes nitrogen inside the root nodules of legumes B1 leghaemoglobin, supplied by the plant, mops up oxygen inside the nodule and protects the bacterium's nitrogenase