How is nitrogen recycled by bacteria, and what controls how fast dead matter decays?
The nitrogen cycle and the roles of nitrogen-fixing, nitrifying, decomposing and denitrifying bacteria, the process of decomposition, and the factors affecting the rate of decay (temperature, water and oxygen).
A focused answer to the OCR Gateway GCSE Biology A topic B4 on the nitrogen cycle and decomposition, covering nitrogen-fixing, nitrifying, decomposing and denitrifying bacteria, the process of decay, and the factors affecting the rate of decomposition.
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What this dot point is asking
OCR wants you to describe the nitrogen cycle and the roles of the four types of bacteria, explain why nitrogen matters to plants, describe decomposition, and explain the factors affecting the rate of decay.
Why nitrogen matters
Living things need nitrogen to make amino acids, and therefore proteins, and to make DNA. The air is about 78 percent nitrogen gas, but this gas is very unreactive, so plants and animals cannot use it directly. Instead, plants take in nitrates from the soil through their roots and use them to build amino acids; animals get their nitrogen by eating plants (or other animals). The nitrogen cycle keeps nitrogen moving between the air, the soil and living things.
The four types of bacteria
Four groups of bacteria drive the nitrogen cycle. Learn what each one does:
- Nitrogen-fixing bacteria convert nitrogen gas from the air into nitrogen compounds (ammonia or nitrates). Some live free in the soil; others live in the root nodules of plants such as peas and beans (legumes).
- Decomposers (decomposing bacteria and fungi) break down dead organisms and waste, releasing ammonia into the soil.
- Nitrifying bacteria convert ammonia (ammonium compounds) into nitrates, the form plants can absorb.
- Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air. This reduces the nitrate available to plants, and happens most in waterlogged, airless soils.
Decomposition
Decomposition is essential: without it, dead bodies and waste would pile up and the nutrients (such as nitrogen) would be locked away instead of being recycled.
Factors affecting the rate of decay
Decomposers work fastest in certain conditions, because they are living things whose enzymes and respiration depend on the environment:
- Temperature. Warmer conditions speed up decay, because the decomposers' enzymes work faster (up to an optimum). Very high temperatures denature the enzymes and very low temperatures slow them right down, which is why a fridge or freezer preserves food.
- Water (moisture). Moist conditions speed up decay, because microorganisms need water to live and to carry out reactions. Dry conditions slow decay, which is why drying preserves food.
- Oxygen. Most decomposers respire aerobically, so plenty of oxygen (good aeration) speeds up decay. Airless conditions slow it down, and in waterlogged soils decay is slow.
Gardeners use these ideas in a compost bin: warm, moist, well-aerated compost decays quickly, recycling nutrients for the garden.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksDescribe the roles of the four types of bacteria in the nitrogen cycle, and explain why nitrogen is important to plants.Show worked answer →
A 6-mark extended response. Mark it for the four bacteria plus the importance of nitrogen.
Nitrogen-fixing bacteria convert nitrogen gas from the air into ammonia or nitrogen compounds (some live in the soil, some in the root nodules of legumes). Decomposing bacteria (decomposers) break down dead organisms and waste, releasing ammonia. Nitrifying bacteria convert ammonia (ammonium compounds) into nitrates. Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air (this reduces the nitrate in the soil).
Importance: plants absorb nitrates from the soil and use them to make amino acids and then proteins, which are needed for growth. Markers reward all four bacteria with the correct conversions and the point that plants need nitrates to make proteins.
OCR 20214 marksA gardener puts grass cuttings and vegetable peelings into a compost bin. Explain how the conditions in the bin affect how quickly the material decays, referring to temperature, water and oxygen.Show worked answer →
A 4-mark question on decay.
Decomposition is carried out by microorganisms (decomposers), which work fastest in warm, moist conditions with plenty of oxygen.
Temperature: a warmer bin speeds up decay because the microorganisms' enzymes work faster (up to an optimum). Water: moist conditions are needed because microorganisms need water to live and to carry out reactions, so a damp bin decays faster than a dry one. Oxygen: most decomposers respire aerobically, so good aeration (oxygen) speeds up decay; airless conditions slow it down. Markers reward the effect of each of the three factors on the activity of the decomposers.
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