Nutrient Cycles

Question 1

Role of saprobionts

Answer 1

• Decomposers (feed on the remains of dead plants and animals and on their waste products, breaking them down)
• Extracellular digestion (secrete enzymes and digest their food externally)
• Saprobiotic nutrition (absorb the nutrients they need)

Question 2

What are myocorrhizae?

Answer 2

Fungi made of long thin strands (hyphae) that form a symbiotic relationship with plants by increasing surface area of the roots. Helps plant absorb rare ions and water and fungi obtains organic compounds

Question 3

The nitrogen cycle

Answer 3

• Nitrogen gas undergoes nitrogen fixation (nitrogen fixing bacteria in root nodules of legumes convert nitrogen into ammonia, forming ammonium ions)
• Nitrification (nitrifying bacteria oxidise ammonium ions in the soil into nitrites then nitrates)
• Some nitrates taken up and used by plants
• Plants / animals die and ammonification takes place (nitrogen compounds from dead organisms converted into ammonia by sapriobionts, forming ammonium ions)
• Other nitrates undergo denitrification (denitrifying bacteria converts nitrates into nitrogen gas in the atmosphere under anaerobic conditions)

Question 4

The phosphorus cycle

Answer 4

• Phosphate ions in rocks are released into the soil / lake by weathering
• Phosphate ions are taken into the plants through the roots
• Phosphate ions are transferred through the food chain as animals eat the plants
• Phosphate ions are lost from the animals in waste products
• Plants and animals die and saprobionts break down the organic compounds, releasing phosphate ions into the soil for assimilation by plants

Question 5

Benefits of natural fertilisers

Answer 5

• Aerate soil
• Contain a wider range of elements
• Consume less energy
• Less leaching

Question 6

What is leaching?

Answer 6

When soluble compounds are washed off farmland by rain (more common in artificial fertilisers)

Question 7

Eutrophication

Answer 7

• Nitrate / phosphate ions leached into fresh water
• Algal bloom forms, blocking out light
• Plants can’t photosynthesise and die
• Saprobionts decompose dead plants, respiring aerobically
• Not enough oxygen so fish die

Question 8

Why is fertiliser used?

Answer 8

Harvesting crops removes nitrates and phosphates from their cycles, depleting soil of nitrate and phosphate