5.4 - Nutrient cycles

Name the general stages in the

phosphorus cycle

1. Weathering

2. Runoff

3. Assimilation

4. Decomposition

5. Uplift

Why is the phosphorus cycle a slow

process?

● Phosphorus has no gas phase, so

there is no atmospheric cycle.

● Most phosphorus is stored as PO 4

3- in

rocks.

What happens during weathering and

runoff?

Phosphate compounds from sedimentary

rocks leach into surface water and soil.

Explain the significance of phosphorus to

living organisms.

Plants convert inorganic phosphate into

biological molecules e.g. DNA, ATP, NADP…

Phosphorus is passed to consumers via

feeding.

What happens during uplift?

Sedimentary layers from oceans (formed

by the bodies of aquatic organisms) are

brought up to land over many years.

How does mining affect the phosphorus

cycle?

Speeds up uplift.

Name the 4 main stages of the nitrogen

cycle.

1. Nitrogen fixation

2. Ammonification

3. Nitrification

4. Denitrification

Why can’t organisms use nitrogen

directly from the atmosphere?

N 2 is very stable due to strong covalent

triple bond.

What happens during atmospheric

fixation of nitrogen?

1. High energy of lightning breaks N 2 into

N.

2. N reacts with oxygen to form NO 2

- .

3. NO 2

- dissolves in water to form NO 3

- .

Outline the role of bacteria in nitrogen

fixation.

Mutualistic nitrogen-fixing bacteria in

nodules of legumes & free-living bacteria

in soil.

Use the enzyme nitrogenase to reduce

gaseous nitrogen into ammonia.

Outline the role of bacteria in

ammonification.

1. Saprobionts feed on and decompose

organic waste containing nitrogen (e.g.

urea, proteins, nucleic acids…).

2. NH 3 released.

3. NH 3 dissolves in water in soil to form

NH 4

+ .

Outline the role of bacteria in nitrification.

2-step process carried out by

saprobionts in aerobic conditions:

2NH 4

+ + 3O 2 → 2NO 2

- + 2H 2 O + 4H +

2NO 2

- + O 2 → 2NO 3

-

Outline the role of bacteria in

denitrification.

Anaerobic denitrifying bacteria convert

soil nitrates back into gaseous nitrogen

Explain the significance of nitrogen to

living organisms.

Plant roots uptake nitrates via active transport &

use them to make biological compounds e.g:

● amino acids

● NAD/ NADP

● nucleic acids

Outline the role of mycorrhizae.

Mutualistic relationship between plant

and fungus increases surface area of

root system = increases uptake of water

and mineral ions.

Give 3 benefits of planting a different

crop on the same field each year.

● Nitrogen-fixing crops e.g. legumes make soil

more fertile by increasing soil nitrate content.

● Different crops have different pathogens.

● Different crops use different proportions of

certain ions.

Name the 2 categories of fertiliser and

state the purpose of using fertiliser.

● Organic: decaying organic matter & animal

waste.

● Inorganic: minerals from rocks, usually

containing nitrogen, phosphorus, potassium.

● To increase gross productivity for higher yield.

At a certain point, using more fertiliser no

longer increases crop yield. Why?

A factor unrelated to the concentration of

mineral ions limits the rate of

photosynthesis, so rate of growth cannot

increase any further.

Outline 2 main environmental issues

caused by the use of fertilisers.

1. Leaching: nitrates dissolve in rainwater

and ‘runoff’ into water sources.

2. Eutrophication: water source becomes

putrid as a result of algal bloom.

What happens during eutrophication?

1. Aquatic plants grow exponentially since nitrate level is no

longer a limiting factor.

2. Algal bloom on water surface prevents light from

reaching the bottom and plants die.

3. Oxygen levels decrease as population of aerobic

saprobionts increases to decay dead matter, so fish die.

4. Anaerobic organisms reproduce exponentially and

produce toxic waste which makes water putrid.

How can the risk of eutrophication be

reduced?

● Sewage treatment marshes on farms.

● Pumping nutrient-enriched sediment

out of water.

● Using phosphate-free detergent.