Unit 3 Nutrient cycles
Key unit competence: Analyse the water, carbon, phosphorus and nitrogen cycles and explain their importance in ecosystems.
Key ideas
- Nutrient (biogeochemical) cycles move chemical elements between living things and the non-living environment (air, water, soil, rocks). Energy flows one way, but matter is recycled; decomposers are essential to every cycle.
- Water cycle: water evaporates from oceans, lakes and soil, and transpires from leaves; water vapour condenses to form clouds; precipitation (rain) falls; water runs off into rivers, infiltrates soil and becomes groundwater, and is taken up by roots and drunk by animals. Forests increase transpiration and local rainfall.
- Carbon cycle: photosynthesis removes CO₂ from the air and builds it into organic compounds; carbon passes along food chains; respiration by plants, animals and decomposers returns CO₂; combustion of fuels and wood releases CO₂; dead remains buried for millions of years form fossil fuels (coal, oil, gas).
- Burning fossil fuels and deforestation increase CO₂ in the air, strengthening the greenhouse effect and causing global warming and climate change.
- Nitrogen cycle: air is 78% nitrogen (N₂), which most organisms cannot use. Nitrogen fixation changes N₂ into usable compounds: by free-living bacteria (Azotobacter), symbiotic Rhizobium in legume root nodules, lightning, and the industrial Haber process (fertilisers).
- Plants absorb nitrates to make proteins; animals eat plants. Decomposers (putrefying bacteria and fungi) turn proteins and urea in dead matter and wastes into ammonium (ammonification). Nitrifying bacteria convert ammonium to nitrites (Nitrosomonas) and nitrites to nitrates (Nitrobacter). Denitrifying bacteria in waterlogged, airless soils change nitrates back to N₂ gas.
- Farmers improve soil nitrogen by growing legumes (beans, peas, groundnuts, clover), crop rotation, adding manure and compost, and ploughing and draining soil (air helps nitrifying bacteria and stops denitrification).
- Phosphorus cycle: phosphorus has no gas phase. Phosphate is released from weathering of rocks, absorbed by plants (for DNA, ATP, cell membranes), passed to animals, returned by decomposers; some washes into lakes and forms sediments, slowly becoming rock again. Fertilisers add phosphates; excess causes eutrophication.
- Importance of cycles: they recycle limited elements, keep soil fertile, and balance the composition of air and water. Human activities (burning fuels, deforestation, overuse of fertilisers, draining wetlands) disturb them.
Common misconceptions
- "Plants absorb nitrogen gas from the air through their leaves." No: most plants absorb nitrates from the soil; only bacteria can fix N₂.
- "Denitrifying bacteria are useful to farmers." No: they remove nitrates from soil, lowering fertility; they thrive in waterlogged soil.
- "The carbon cycle only involves photosynthesis and respiration." No: combustion, decomposition and fossil fuel formation are also parts of it.
- "Phosphorus is cycled through the air like nitrogen." No: the phosphorus cycle has no gaseous stage; it cycles through rocks, soil, water and organisms.
Worked examples
Example 1. Explain why farmers plant beans before maize on the same land.
Solution
Beans are legumes with Rhizobium in root nodules that fix nitrogen. When bean roots and leaves decay, nitrogen compounds are released and converted to nitrates. The following maize crop uses these nitrates to make proteins and grows better with less fertiliser.
Example 2. Explain how a dead leaf's nitrogen becomes available to a growing plant.
Solution
Decomposers break down proteins in the leaf into ammonium (ammonification). Nitrosomonas oxidises ammonium to nitrite, and Nitrobacter oxidises nitrite to nitrate (nitrification). Roots absorb the nitrate by active transport and use it to make amino acids and proteins.
Practice questions
Describe the main processes of the water cycle.
Show answer
Evaporation from water bodies and soil; transpiration from plants; condensation of vapour into clouds; precipitation as rain; run-off to rivers and infiltration into soil and groundwater; uptake by plants and animals.
Common mistake: Leaving out transpiration.
Describe how carbon is cycled in nature.
Show answer
Photosynthesis takes CO₂ from air into organic compounds; feeding passes carbon along food chains; respiration of all organisms and decomposers returns CO₂; combustion releases CO₂; some remains form fossil fuels which release CO₂ when burnt.
Common mistake: Missing respiration of decomposers.
Name three ways in which nitrogen is fixed.
Show answer
Free-living bacteria (Azotobacter); symbiotic Rhizobium in legume nodules; lightning; industrial Haber process.
Common mistake: Calling nitrification fixation.
Explain the roles of nitrifying and denitrifying bacteria.
Show answer
Nitrifying bacteria (Nitrosomonas, Nitrobacter) convert ammonium to nitrites then nitrates, which plants absorb. Denitrifying bacteria convert nitrates into nitrogen gas, removing them from soil.
Common mistake: Saying both increase soil nitrates.
Give three ways farmers can increase soil nitrogen.
Show answer
Grow legumes / crop rotation with legumes; add manure or compost; add nitrogen fertilisers; plough and drain soil to aerate it.
Common mistake: Suggesting flooding fields.
Explain how human activities increase carbon dioxide in the atmosphere.
Show answer
Burning fossil fuels in vehicles and factories, burning wood and charcoal, and deforestation (fewer trees to absorb CO₂ and burning releases it).
Common mistake: Saying photosynthesis increases CO₂.
Describe the phosphorus cycle.
Show answer
Weathering releases phosphate from rocks into soil and water; plants absorb it; animals eat plants; decomposers return phosphate from wastes and dead bodies; some washes to lakes and seas and forms sediments that become rock.
Common mistake: Including a gas stage.
State two importances of nutrient cycles.
Show answer
They recycle limited elements so they are available to living things again; keep soil fertile; maintain balanced levels of gases such as CO₂ and O₂.
Common mistake: Saying cycles create new energy.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The phosphorus cycle has no: a. condensation 2. Formation of clouds from water vapour is: b. gaseous stage 3. Plants absorb nitrogen mainly as: c. nitrates 4. Water vapour lost from leaves is: d. transpiration Show answer
1 → b (gaseous stage) · 2 → a (condensation) · 3 → c (nitrates) · 4 → d (transpiration)
- Phosphates move through rocks, soil and organisms.
- Vapour cools and becomes droplets.
- Nitrates dissolve in soil water.
- It is part of the water cycle.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
78% · lost as heat · recycled · algal blooms and fish deaths- Excess phosphate fertiliser washing into Lake Muhazi can cause .
- The percentage of nitrogen in air is about .
- Matter in an ecosystem is .
Show answer
1: algal blooms and fish deaths · 2: 78% · 3: recycled
- This is eutrophication.
- Oxygen is about 21%.
- Elements move between living and non-living parts.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Changing nitrogen gas into nitrogen compounds is: a. nitrogen fixation 2. Coal and oil are formed from: b. weathering of rocks 3. Phosphate is released naturally from: c. remains of organisms buried for ages 4. Plants use phosphorus to make: d. DNA and ATP Show answer
1 → a (nitrogen fixation) · 2 → c (remains of organisms buried for ages) · 3 → b (weathering of rocks) · 4 → d (DNA and ATP)
- Bacteria and lightning fix nitrogen.
- They are fossil fuels.
- Rain slowly dissolves rocks.
- It is also in cell membranes.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
photosynthesis · root nodules of legumes · decomposition · nitrogen gas- Denitrifying bacteria change nitrates into .
- The process that removes carbon dioxide from the air is .
- Rhizobium bacteria live in .
Show answer
1: nitrogen gas · 2: photosynthesis · 3: root nodules of legumes
- They live in airless soils.
- CO₂ is built into glucose.
- They fix nitrogen in mutualism.