The Water and Nitrogen Cycles

Carbon is not the only material recycled through ecosystems. Water is endlessly evaporated, carried through the air and returned as rain, providing the fresh water on which all life depends. And nitrogen, needed by every organism to make proteins and DNA, is passed between the air, the soil and living things through a cycle driven largely by bacteria. This lesson, part of Topic B4 of OCR Gateway Science A, traces both cycles. The water cycle is fairly intuitive; the nitrogen cycle is more involved, with four different types of bacteria each playing a distinct role, so we will build it up carefully.

By the end of this lesson you should be able to describe the stages of the water cycle, explain why organisms need nitrogen, and describe the nitrogen cycle including the roles of nitrogen-fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria, together with the parts played by lightning and legumes.

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The Water Cycle

The water cycle describes how water is continually recycled between the oceans, the air, the land and living things, providing the fresh water that plants and animals need. It is powered by energy from the Sun.

The stages are:

1. Evaporation. The Sun's heat evaporates water from the surface of the seas, rivers and lakes, turning liquid water into water vapour that rises into the air. Transpiration — the evaporation of water from the leaves of plants — also adds water vapour to the air.
2. Condensation. As the water vapour rises and cools, it condenses into tiny droplets, forming clouds.
3. Precipitation. When the droplets join and grow heavy enough, they fall as precipitation — rain, snow, sleet or hail.
4. Return. The water that falls on land soaks into the soil, is taken up by plants, or flows in rivers back to the sea, where the cycle begins again.

This cycle provides the fresh water (not salty) that land organisms depend on: when sea water evaporates, the salt is left behind, so the rain that falls is fresh.

Exam Tip: Use the four key terms in order — evaporation → condensation → precipitation → (return/runoff) — and remember that transpiration from plants also releases water vapour. A common mark is for noting that the water cycle provides fresh water because the salt is left behind when sea water evaporates.

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Why Organisms Need Nitrogen

Around 78% of the air is nitrogen gas (N₂), yet plants and animals cannot use it in this form. Nitrogen is essential because it is needed to make proteins (for growth and repair) and DNA — both contain nitrogen. The problem is that nitrogen gas is very unreactive: its two atoms are held together so strongly that most organisms cannot break them apart to use the nitrogen. So although there is nitrogen everywhere in the air, organisms must obtain it in a usable form, and this is where the nitrogen cycle — and especially its bacteria — comes in.

• Plants absorb nitrogen as nitrate ions (NO₃⁻) from the soil through their roots, and use it to make proteins and DNA.
• Animals get their nitrogen by eating plants (or other animals) and digesting their proteins.

So the whole cycle is really about converting unusable nitrogen gas into nitrates that plants can absorb, passing that nitrogen through living things, and eventually returning it to the air.

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The Nitrogen Cycle

Four types of bacteria drive the nitrogen cycle, each carrying out a distinct conversion. Learning which bacterium does what is the key to this topic.

1. Nitrogen-fixing bacteria

Nitrogen-fixing bacteria convert nitrogen gas (N₂) from the air into nitrogen compounds (nitrates) that plants can use — a process called nitrogen fixation. These bacteria live free in the soil and also in root nodules (small lumps) on the roots of legumes (plants such as peas, beans and clover). The relationship benefits both: the bacteria get sugars from the plant, and the plant gets a built-in supply of usable nitrogen. This is why farmers grow legumes to enrich the soil with nitrogen.

2. Decomposers (ammonification)

When plants and animals die, and when animals produce waste (urine and faeces), the nitrogen in their proteins is returned to the soil by decomposers (bacteria and fungi). The decomposers break down the proteins and release the nitrogen as ammonia (which forms ammonium compounds). This stage is sometimes called ammonification. The decomposers thus put nitrogen back into the soil — but as ammonia, not yet as the nitrates plants need.

3. Nitrifying bacteria

Nitrifying bacteria convert the ammonia (from decay) into nitrates in the soil — the form plants can absorb. This process is called nitrification. Because nitrifying bacteria need oxygen, this conversion works best in well-aerated soil — which is one reason farmers plough and drain their land. Nitrifying bacteria complete the loop that makes decayed nitrogen available to plants again.

4. Denitrifying bacteria

Denitrifying bacteria do the opposite of nitrogen fixation: they convert nitrates in the soil back into nitrogen gas (N₂), which returns to the air. This process, denitrification, removes usable nitrogen from the soil, so it reduces soil fertility. Denitrifying bacteria thrive in waterlogged, low-oxygen soils — which is another reason farmers drain and plough their fields, to discourage these bacteria and keep nitrates in the soil.

The role of lightning

Nitrogen-fixing bacteria are not the only natural way nitrogen gas becomes usable. Lightning carries so much energy that it can make nitrogen and oxygen in the air react together to form nitrogen oxides, which dissolve in rain and reach the soil as nitrates. Lightning therefore also fixes nitrogen, though it contributes far less than the bacteria.