12 min readPathway sheetSection A MCQ (1 mark) and objective (2 marks, two parts); Section B very short answer (2 marks) or short answer (3 marks) with a labelled diagram

Nutrient cycles: nitrogen, carbon and water

Nutrients such as nitrogen, carbon and water move in cycles between living things, soil, water and air. Unlike energy, they are never lost but used again and again.

Biogeochemical (nutrient) cycleThe cyclic movement of an element such as nitrogen or carbon from soil, water and air into living things and back

Five steps of the nitrogen cycleNitrogen fixation, assimilation, ammonification, nitrification, denitrification

Key bacteriaRhizobium (root nodules of legumes) fixes nitrogen; Nitrosomonas and Nitrobacter carry out nitrification; Pseudomonas carries out denitrification

Carbon in and out of the airTaken out by photosynthesis; put back by respiration, decay, and burning of wood and fuels

Energy flow vs nutrient cycling

Energy flowOne way only: sun → producers → consumers, with energy lost as heat at each step. It is not reused.

Nutrient cyclingCyclic: nutrients pass from soil, water and air into living things and back again. They exist in fixed amounts and are never lost.

Nature keeps refilling each nutrient in a cycle, which is why we say 'what goes around comes around'.

The nitrogen cycle in five steps

Air is the biggest store of nitrogen, but most living things cannot use nitrogen gas directly. Nitrogen is needed to make proteins and nucleic acids.

1★
Nitrogen fixation: free nitrogen gas is changed into nitrites and nitrates. Lightning makes nitrogen and oxygen combine; the oxides dissolve in rain and reach the soil. Microbes also fix nitrogen. These are free-living blue-green algae and bacteria in soil, and bacteria in the root nodules of leguminous plants (pulses such as pea and gram).
Root nodule bacterium: Rhizobium. The answer key in your NIOS lesson calls this step ammonification; the correct name is nitrogen fixation.
2
Nitrogen assimilation: plants absorb nitrates from soil and water to make amino acids and proteins. Animals get this nitrogen as proteins when they eat plants.
3
Ammonification: proteins in animals are broken into urea and ammonia, which leave the body in urine and excreta. Dead plants and animals return nitrogen as ammonium compounds, which ammonifying bacteria turn into ammonia.
4★
Nitrification: nitrifying bacteria in soil change ammonia into nitrites (Nitrosomonas), and then nitrites into nitrates (Nitrobacter). Plants take up the nitrates again.
Nitrosomonas and Nitrobacter (your NIOS lesson prints Nitrosobacter)
5
Denitrification: denitrifying bacteria in soil change nitrates and nitrites back into nitrogen gas, which returns to the air.
Pseudomonas. Your lesson also names Clostridium, which is better known as a free-living bacterium that fixes nitrogen; give Pseudomonas as your example.

★ key step

The carbon cycle

Carbon moves between the air, water, soil and living things. It is the most important cycle on Earth.

1★
Photosynthesis: green plants and algae take carbon dioxide from air and water and build it into food (carbohydrates).
2
Food chains: animals get carbon by eating plants, or other animals.
3
Respiration: plants, animals and microbes break down food and give out carbon dioxide into the air.
4
Decay: decomposers break down dead bodies and waste, releasing carbon dioxide.
5
Fossil fuels: some dead remains buried over millions of years became coal, petroleum and natural gas.
6★
Burning: burning wood and fossil fuels in homes, vehicles and factories returns their carbon to the air as carbon dioxide.

★ key step

Draw it: the water cycle

back to Water in oceans, rivers and lakes

Plants also return water vapour to the air through their leaves (transpiration), and animals do so by breathing and sweating. The Earth's water is limited, so it is used again and again.

How humans affect the cycles

Burning coal, petrol, diesel and wood in factories, vehicles and homes adds carbon dioxide to the air.
Cutting forests leaves fewer trees to take in carbon dioxide, so its level in the air rises.
You return carbon dioxide to the air when you breathe out, and nitrogen to the soil through urine and excreta.
Vehicles and factories add oxides of nitrogen to the air; too much nitrogen fertiliser washes into ponds.
Growing legumes (pulses) adds nitrogen to soil naturally and cuts the need for fertiliser.

How to draw the nitrogen cycle

At the top, write Nitrogen gas (N₂) in the air.
Draw a lightning flash and a legume root with nodules below it, with arrows down labelled fixation (Rhizobium).
Below, write Nitrates in soil, with an arrow to a plant labelled assimilation, and an arrow from the plant to an animal labelled eaten.
From the plant and animal, draw arrows to Dead matter and excreta, then to Ammonia, labelled ammonification.
From ammonia, draw an arrow back to Nitrates, labelled nitrification (Nitrosomonas, Nitrobacter).
From nitrates, draw an arrow up to the air, labelled denitrification (Pseudomonas).

In real life

Farmers in India grow pulses such as moong, chana and arhar between cereal crops. Rhizobium in their root nodules adds nitrogen to the soil and saves chemical fertiliser.

Exam angle

1 marks: “Which process returns nitrogen gas from the soil to the air?” Denitrification, by bacteria such as Pseudomonas.

3 marks: “Follow a carbon atom from the carbon dioxide in the air into a cow and back into the air. Then explain how more vehicles and the clearing of forests change the carbon dioxide in the air.” Grass takes in CO₂ by photosynthesis and builds it into food; the cow eats the grass; the cow's respiration, and the decay of its dung and body, return CO₂ to the air. Vehicles burn fuels and add CO₂; cleared forests leave fewer plants to take it in, so CO₂ rises.

3 marks: “Gram plants cannot use nitrogen gas, yet they get all the nitrogen they need. Trace how nitrogen from the air reaches a gram plant, then an animal, and returns to the air, naming a bacterium for any two steps.” Fixation: Rhizobium in gram root nodules (or lightning) turns N₂ into nitrogen compounds. Assimilation: the plant makes proteins; an animal eats it. Ammonification: excreta and dead bodies give ammonia. Nitrification: Nitrosomonas, Nitrobacter make nitrites and nitrates. Denitrification: Pseudomonas returns N₂ to the air.

Remember it

Fix, Absorb, Rot, Nitrify, Return: fixation, assimilation, ammonification, nitrification, denitrification. Energy flows; nutrients cycle.