Nutrient cycles

Question 1

Why is it important that nutrients are recycled?

Answer 1

Because there is a limited availability of nutrient ions in a usable form. It is important therefore that nutrients such as carbon, nitrogen and phosphorus are recycled - the flow is cyclic rather than linear

Question 2

What is the simple sequence of a nutrient cycle?

Answer 2

• nutrients are taken up by producers as simple, organic molecules
• the producer incorporates the nutrient into complex organic molecules
• when the producer is eaten, the nutrient passes into consumers
• nutrient passes along the food chain as consumers are eaten
• when producers and consumers die, they are broken down by saprobiontic microorganisms which release the nutrient in its simple, original form

Question 3

What would happen without saprobionts?

Answer 3

Nutrients would remain locked up as part of complex molecules that cannot be taken up or used by plants again

Question 4

Why are nitrates needed in plants?

Answer 4

• manufacture amino acids, proteins and nucleic acids needed for growth

Question 5

How do plants take up nitrate ions?

Answer 5

Through active transport in the soil

Question 6

What are the 4 main stages in the nitrogen cycle?

Answer 6

• ammonification
• nitrification
• nitrogen fixation
• denitrification

Question 7

What is ammonification?

Answer 7

nitrogen compounds in waste products and dead organisms are converted into ammonia by saprobionts

Question 8

What is nitrification?

Answer 8

Ammonium ions in soil are converted by nitrifying bacteria into nitrite ions (NO2-) and then nitrate ions (NO3-), which are nitrogen compounds that can be used by plants

Question 9

What is nitrogen fixation?

Answer 9

Nitrogen gas is converted into nitrogen containing compounds by nitrogen-fixing bacteria

• bacteria reduce gaseous nitrogen to ammonia, which they use to manufacture amino acids. Nitrogen rich compounds are released when they die

Question 10

What is denitrification?

Answer 10

when soil becomes waterlogged, there is an increase in anaerobic gentrifying bacteria. These convert soil nitrates into gaseous nitrogen during respiration, which reduces the availability of nitrogen containing compounds for plants. (therefore soil must be kept aerated to reduce denitrifying bacteria)

Question 11

Why is phosphorus important?

Answer 11

Component of ATP, phospholipids and nucleic acids - life therefore depends on it being constantly recycled

Question 12

How are nitrates made available to plants?

Answer 12

• proteins are converted into ammonium compounds by saprobionts
• ammonium converted to nitrite, which is converted into nitrate by nitrifying bacteria
• nitrate is converted to ammonium by nitrogen fixing bacteria

Question 13

What are the stages of the phosphorus cycle?

Answer 13

• phosphate ions found mostly in the form of sedimentary rocks - erosion and weathering of these rocks helps phosphate ions to become dissolved
• available for absorption, so taken up by plants (assimilation)
• phosphate ions pass to animals which feed on plants
• after death and excretion, decomposers break them down and release phosphate ions into the water/soil
• phosphate ions in streams/rivers are transported to oceans where sedimentation occurs

Question 14

What are mycorrhizae and what is their role?

Answer 14

• fungi which act like extensions of the plants root system, which vastly increases surface area for the absorption of water and minerals
• plants benefit from improved water and ion uptake whist fungus receives organic compounds such as sugars and amino acids from the plant

Question 15

Why are fertilisers used?

Answer 15

• in agricultural systems, crops are harvested meaning that nutrients cannot be recycled - necessary to replenish them so that they will become a limiting factor to plant growth, reducing productivity and yield

Question 16

What are the 2 types of fertiliser?

Answer 16

• natural (organic) - consist of dead and decaying remains of plants/animals
• artificial (inorganic) - pure chemical compounds consisting of nitrogen, phosphorus and potassium

Question 17

Why should you use a specific amount of fertiliser?

Answer 17

• increasing fertiliser will add more nitrogen ions to the soil which are used to create more protein - this increases plant biomass and leaf area, increasing the rate of photosynthesis and crop productivity
• however too much fertiliser will cause the plants water potential to decrease - this means less water will be absorbed by the plant which reduces the rate of photosynthesis

Question 18

What are the advantages and disadvantages of organic fertilisers?

Answer 18

Advantages:
- improves soil structure which reduces erosion and increases water holding ability
- nutrients are released over a long period of time - applied less often
- less likely to cause water pollution
- cheaper and more easily available

Disadvantages:
- nutrients are not very concentrated so large amounts needed
- biohazard - may contain pathogens
- exact nutrient composition is not known

Question 19

What are the advantages and disadvantages of inorganic fertilisers?

Answer 19

Advantages:
- exact chemical composition is known so easier to know how much to apply, and what the effect on yield will be
- concentrated so smaller amounts needed - reduced transport costs
- easy to apply evenly and are clean - easy to handle

Disadvantages:
- obtained from mines/quarries - more expensive
- highly soluble - water pollution
- favour fast growing species - decreases biodiversity

Question 20

What is leaching?

Answer 20

• nutrients are removed from the soil and dissolved by rainwater
• they may drain into freshwater lakes from rivers/streams causing water pollution and eutrophication

Question 21

What is eutrophication and why is it bad?

Answer 21

the process by which nutrient concentrations increase in bodies of water

• water becomes over enriched with nutrients from fertilisers
• this causes aquatic plant growth as nitrate ion concentration increases
• as algae mostly grows on the surface, the upper water layers become densely populated with algae, called an ‘algal bloom’
• this prevents light from penetrating to lower depths, so light becomes a limiting factor for plants and algae beneath the surface, causing them to die
• saprobiontic organism population grows due to increased dead matter as a food source
• saprobiontic organisms require oxygen for respiration - oxygen concentration decreases
• this causes aerobic organisms such as fish to die due to low oxygen
• less competition for anaerobic organisms, so populations rise and they further decompose dead organisms which releases more nitrates and some toxic material