OxfordAQA International AS and A-level Biology: Nutrient cycles: carbon and nitrogen (9610)

This guide covers section 3.3.5 Nutrient cycles (3.3.5.1 to 3.3.5.3) of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. Section 3.3.5 sits in Unit 3, Populations and genes, which the specification lists as International A-level only. The specification asks you to analyse, interpret and evaluate data in this section, and the worked examples below practise that.

Use it with the revision notes and the practice questions. Course hub: OxfordAQA A-level Biology. Checklist: OxfordAQA A-level Biology checklist. To find weak spots first, try the free diagnostics.

What the section covers

Section	What you must be able to do
3.3.5.1	Explain that elements in organic substances are released by decomposers and saprophytic microorganisms as ions or simple inorganic molecules, taken up by producers, and passed along food chains by digestion and assimilation
3.3.5.2	Explain how respiration, photosynthesis and human activity cause short-term fluctuations and long-term change in global carbon dioxide concentration; explain the roles of carbon dioxide and methane in the greenhouse effect and climatic change
3.3.5.2 skills	Analyse, interpret and evaluate data on short-term carbon dioxide fluctuations, and on effects of climate change on crop yield and on insect pest life cycles and numbers
3.3.5.3	Describe the role of bacteria in ammonification, nitrification, denitrification and nitrogen fixation; explain the use of natural and artificial fertilisers; explain leaching and eutrophication

3.3.5.1 Principles of nutrient cycling

Living things are built from organic substances that contain elements such as carbon, hydrogen, oxygen, nitrogen and phosphorus. The supply of each element on Earth is fixed, so it must be used again and again.

Decomposers and saprophytic microorganisms (bacteria and fungi that feed on dead organic matter) do the recycling. A saprophyte feeds like this:

1. It secretes digestive enzymes onto dead material, so digestion happens outside its cells.
2. The enzymes hydrolyse big insoluble molecules, for example proteins, into amino acids and other soluble products.
3. The saprophyte absorbs what it needs, and respires some of it.

Respiration releases carbon dioxide. Breakdown of proteins and nucleic acids releases ammonium ions. These are the ions and simple inorganic molecules in the specification. Producers take them up and build them into new organic molecules.

Elements move between consumers by digestion (large molecules hydrolysed to small soluble ones that are absorbed) and assimilation (the absorbed molecules become part of the consumer’s own cells). For example, a rabbit digests plant protein to amino acids, then assimilates them into its own proteins. When the rabbit dies, saprophytes return its nitrogen to the soil.

3.3.5.2 The carbon cycle

Carbon enters living things when producers fix carbon dioxide in photosynthesis. It passes along food chains as organic compounds. It returns to the air as carbon dioxide by respiration of producers, consumers and decomposers. Burning fossil fuels and wood (combustion) also releases carbon dioxide, which is a human activity that moves carbon from long-term stores into the air.

Short-term fluctuations

Photosynthesis removes carbon dioxide only in the light, but respiration goes on all the time. So carbon dioxide concentration varies over a day, and over a year in places with a growing season.

Worked example 1: daily change in a crop canopy

Air at the height of a dense crop had these carbon dioxide concentrations: 468 ppm at 04:00, 410 ppm at 08:00, 352 ppm at 13:00 and 431 ppm at 19:00.

Fall from 04:00 to 13:00 = 468 − 352 = 116 ppm, which is 116 ÷ 468 × 100 = 24.8% of the night value.

At 04:00 there is no light, so no photosynthesis, and respiration by crop, soil organisms and decomposers raises the concentration. By 13:00 photosynthesis runs faster than respiration, so the crop takes in more carbon dioxide than is released. By 19:00 light is falling, photosynthesis slows and the concentration rises again.

Long-term change and human activity

Combustion of fossil fuels moves carbon from deposits formed over a very long time into the air. Clearing forest removes trees that fix carbon dioxide, and burning or decay of the wood releases the carbon. The result is a rise in the average concentration, with the short-term cycles still visible on top.

Worked example 2: seasonal and long-term change

Monthly carbon dioxide concentrations (ppm) at a station in the northern hemisphere:

Month	Year 1	Year 2
January	419.6	421.9
March	420.9	423.2
May	421.8	424.1
July	419.3	421.6
September	416.7	419.0
November	418.2	420.5

Seasonal range in year 1 = 421.8 − 416.7 = 5.1 ppm. Year 2 gives the same, 424.1 − 419.0 = 5.1 ppm.

Rise from year 1 to year 2 is 2.3 ppm in every month, so the annual increase is 2.3 ppm, which is 2.3 ÷ 421.8 × 100 = 0.55% of the May value.

The concentration falls from May to September because the large land areas of the northern hemisphere are in leaf, and photosynthesis exceeds respiration. It rises from September to May as leaves die, photosynthesis falls and decomposers respire the litter. The identical range each year shows a regular seasonal cycle. The steady annual rise shows a long-term increase that the cycle does not explain, and this fits the burning of fossil fuels.

Greenhouse effect and climatic change

Incoming short-wave radiation from the Sun mostly reaches the ground and heats it. The surface re-emits longer-wave infrared radiation. Greenhouse gases, including carbon dioxide and methane, absorb some of this infrared and re-radiate it in all directions, including back down. This keeps the lower atmosphere warmer than it would be otherwise. This is the natural greenhouse effect. More carbon dioxide and methane enhance it, so average temperatures rise, and patterns of rainfall and seasons change. These changes together are climatic change.

Carbon dioxide comes from combustion, respiration and deforestation. Methane comes from anaerobic respiration by methanogenic archaea in waterlogged soils and rice paddies, in the guts of cattle and sheep, and from landfill. Methane also leaks from natural gas supplies. Per molecule, methane absorbs infrared more strongly than carbon dioxide, but there is far less of it.

Effects of climate change on crops and insect pests

You may be asked to analyse, interpret and evaluate data on the effects of climate change on yield of crop plants and on the life cycles and numbers of insect pests.

• Crop yield: the effect depends on the crop and the place. A warmer climate can lengthen a growing season, and more carbon dioxide can raise the rate of photosynthesis. But heat can denature enzymes, drought reduces water for photosynthesis and growth, and pests may increase.
• Insect pests: insects are ectotherms, so warmth speeds development. The life cycle is shorter, there are more generations in a season, more survive the winter and the range spreads to new areas. Numbers of pests may rise and cause more damage.

Worked example 3: warmth, aphids and yield

Over a growing season of 150 days, an aphid took 15 days to complete a generation at a mean of 15 °C and 10 days at 20 °C. Mean wheat yield in two seasons was 6.4 t ha⁻¹ in the cooler one and 5.6 t ha⁻¹ in the warmer one.

Generations: 150 ÷ 15 = 10 and 150 ÷ 10 = 15, which is 5 more, or 50% more.

Yield change = 5.6 − 6.4 = −0.8 t ha⁻¹, and −0.8 ÷ 6.4 × 100 = −12.5%.

Two seasons cannot establish cause. Rainfall, disease or sowing date may also differ. Fair conclusions are that the warmer season had more pest generations and a lower yield, and that more seasons with measured pest numbers are needed.

3.3.5.3 The nitrogen cycle

Air is about four-fifths nitrogen gas, but producers cannot use it. They take up nitrate (NO₃⁻) and ammonium (NH₄⁺) ions and make amino acids, proteins and nucleic acids. Bacteria carry out the steps below.

Process	Change	Bacteria and conditions
Nitrogen fixation	N₂ to ammonium	Free-living bacteria in soil; mutualistic bacteria in root nodules of legumes, which receive sugars from the plant
Ammonification	Nitrogen in proteins, urea and nucleic acids to ammonium	Saprophytic bacteria (and fungi) on dead matter and waste
Nitrification	Ammonium to nitrite, then nitrite to nitrate	Two groups of nitrifying bacteria; aerobic, so need oxygen
Denitrification	Nitrate to N₂	Denitrifying bacteria; anaerobic, as in waterlogged soil

Fixation and nitrification supply usable nitrogen. Denitrification removes it from the soil, so well-drained soil keeps its fertility better.

Fertilisers

Harvesting plants and removing livestock takes nitrogen out of the system, because the plants and animals never die and decay on that land. Fertilisers replace it.

	Natural	Artificial
Examples	Manure, slurry, compost	Ammonium nitrate, urea, mixed N, P and K products
Nutrients	Organic compounds; saprophytes release ions	Inorganic salts, already soluble
Release	Slow	Fast
Composition	Variable	Known
Other effect	Adds organic matter	None

Worked example 4: how much fertiliser

Urea is CO(NH₂)₂. Using Ar values C = 12, O = 16, N = 14, H = 1, its Mr is 60 and it contains 2 × 14 = 28 of nitrogen.

Nitrogen in urea = 28 ÷ 60 × 100 = 46.7%.

A crop removes 85 kg of nitrogen per hectare. If only 60% of applied nitrogen is taken up, nitrogen to apply = 85 ÷ 0.60 = 141.7 kg per hectare. Urea = 141.7 ÷ 0.4667 = 303.6 kg per hectare, so 2 277 kg for 7.5 hectares.

Leaching and eutrophication

Leaching is the washing of soluble ions, such as nitrate, out of soil by draining water. It is greater after heavy rain just after application, when more is applied than the crop takes up, and on bare or sandy soil.

Eutrophication follows when leached nitrate and phosphate reach water.

1. Nutrient concentration rises, so algae grow quickly and form a bloom.
2. The bloom blocks light, so plants beneath die.
3. Dead material feeds saprophytic bacteria, which multiply and respire aerobically.
4. Dissolved oxygen falls, and fish and other aerobic organisms die.

Worked example 5: reading eutrophication data

Nitrate in a pond rose from 2.6 to 9.8 mg dm⁻³ in a fortnight. Algae rose from 55 to 1 900 cells per cm³. Over the next three weeks oxygen fell from 8.9 to 2.4 mg dm⁻³.

Nitrate: (9.8 − 2.6) ÷ 2.6 × 100 = 277%. Algae: 1 900 ÷ 55 = 34.5 times. Oxygen fall = 6.5 mg dm⁻³, which is 6.5 ÷ 8.9 × 100 = 73%.

The oxygen fall comes after the algae peak because dead algae must first be decomposed by bacteria that use oxygen.

Common errors

• Saying bacteria “make” nitrogen. Fixation converts N₂ to ammonium.
• Swapping ammonification (organic nitrogen to ammonium) and nitrification (ammonium to nitrate).
• Placing denitrification in well-aerated soil.
• Blaming the oxygen fall on algae respiring alone. Decomposer respiration is the main cause.
• Writing “nutrients” where the question needs nitrate, ammonium or carbon dioxide.
• Dividing by the new value in a percentage change.

Official syllabus

This guide follows section 3.3.5 Nutrient cycles in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA.