50 exam-style questions on Biogeochemical cycles — topic 2.3 of Ecology in the IB Environmental Systems and Societies syllabus — with answers and explanations. Samples below; the full set is free with an account.
Which sequence correctly describes nitrification in a well-aerated soil?
Answer: ammonium to nitrite to nitrate
Nitrification is an oxidation carried out by aerobic bacteria, so it runs from the most reduced form upwards to nitrate, the form plant roots take up most readily. Running the sequence backwards describes no real process, and nitrate to nitrogen gas is denitrification.
A paddy field is kept flooded for several weeks and the soil becomes anaerobic. The nitrogen process most likely to increase is
Answer: denitrification, releasing nitrogen gas
Denitrifying bacteria use nitrate instead of oxygen as an electron acceptor, so waterlogging that drives oxygen out of the soil pores favours them. Nitrification needs oxygen and therefore slows down in the same conditions, which is why flooded soils lose available nitrogen to the atmosphere.
The phosphorus cycle differs from the carbon and nitrogen cycles mainly because
Answer: it has no significant gaseous phase
Phosphorus moves through rock, soil, water and organisms but has no meaningful atmospheric reservoir, so it is released only by weathering and returns to rock through sedimentation. That is why the cycle is slow and why mined phosphate is effectively a non-renewable input to agriculture.
A lowland peat bog is drained so that the land can be used for arable farming. In carbon-cycle terms the most likely effect is
Answer: release of carbon to the atmosphere as the peat decomposes
Waterlogging is what preserves peat: it keeps the soil anaerobic so decomposition is slower than plant growth. Draining admits oxygen, aerobic decomposers respire the stored organic matter, and a long-term carbon sink is turned into a source.
Which of these holds by far the largest storage of carbon on Earth?
Answer: Marine sediments and sedimentary rock
Almost all of the planet's carbon is locked in limestone and other sedimentary rock, on timescales of millions of years. The atmospheric storage is tiny by comparison, which is exactly why moving even a small fraction of the fossil storage into it changes the climate so much.
Dead leaves fall onto a woodland soil, and soil organisms convert the nitrogen held in their proteins into ammonium. This step of the nitrogen cycle is
Answer: ammonification
Ammonification releases ammonium from dead organic matter; nitrification then oxidises that ammonium to nitrite and on to nitrate. Both are transformations, because the chemical form of the nitrogen changes at each step.
That was 6 of 50 Biogeochemical cycles questions.
Practise all 50 freeOutlineAO2 · 1–4 marks
A brief account or summary: the main points in two or three sentences, no evaluation.
DistinguishAO2 · 1–4 marks
Make the difference clear for BOTH terms — a definition of one of them is only half an answer.
DrawAO1 · 1–2 marks
A labelled, accurate drawing with a ruler where lines are straight; axes and units on a graph.
50 Biogeochemical cycles questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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