15 nitrogen cycle

Question 1

Nitrogen Fixation

Answer 1

· is carried out by nitrogen fixing bacteria.

· They convert the nitrogen gas (N2) into ammonia/ammonium ions (NH3/NH4+)

· Some of these bacteria live in special swellings on the roots (known as root nodules) of plants called legumes.

· The bacterium receives water and sugars from the plant and in return supplies the plant with ammonia.

· The ammonia can be used to make proteins and other nitrogen containing organic compounds.

· Some of the ammonia produced by the bacteria in root nodules may also end up in the soil.

· There are also free living nitrogen fixing bacteria in the soil. The ammonia that they produce dissolves in water in the soil to form ammonium ions.

· Plants take up nitrogen from the soil in the form of nitrates through their roots by active transport. This is how most of the nitrogen returns to the biotic phase.

Question 2

Decomposition and Ammonification (Saprobiotic nutrition)

Answer 2

· When organisms die, excrete urea or egest faeces, saprobiotic microorganisms (bacteria and fungi) convert the organic nitrogen in these organic compounds into ammonia/ammonium ions (NH3/NH4+) .

· This returns the nitrogen to the abiotic phase.

· The saprobiotic microorganisms (saprobionts) secrete enzymes on to the detritus so the nitrogen-containing compounds (e.g. DNA/proteins/urea) are digested externally.

· This is extracellular digestion .

· The products of digestion are absorbed by the saprobionts, but some ions/molecules (e.g. ammonium ions) are released into the environment.

Question 3

Nitrification

Answer 3

· This process is carried out by nitrifying bacteria. These bacteria convert ammonium ions (NH4+) into nitrates (NO3 - )

· There are two stages of nitrification, each carried out by different types of nitrifying bacteria.

1. The oxidation of ammonium ions (NH4+) to nitrite ions (NO2- ) by nitrifying bacteria.
2. The oxidation of nitrites (NO2- ) to nitrates (NO3 - ) by nitrifying bacteria.

· Plants take up the nitrates through their roots by active transport returning the nitrogen returns to the biotic phase.

Question 4

Denitrification

Answer 4

This is the loss of nitrates from the soil under anaerobic conditions (often in water-logged soil). Nitrates (NO3-) are converted to nitrogen gas (N2 ) . Aerating the soil will reduce this process.

Question 5

The Phosphorus Cycle

Answer 5

· Saprobionts decompose dead animals / plants / algae/ waste materials using extracellular digestion.

· Saprobionts absorb the products of digestion and release phosphate ions (PO43- ) into soils, lakes, rivers and oceans where they dissolve.

· Some of these phosphate ions are absorbed by plants and algae and are used to form organic biological molecules used in growth, such as phospholipids, ATP and nucleic acids.

· The phosphate ions are then passed to the animals that feed on the plants and algae. The phosphate-containing organic compounds are digested then absorbed and assimilated.

· Some of the dissolved phosphates in oceans can, by sedimentation, form rocks.

· The phosphates in sedimentary rocks are only returned to the cycle if erosion occurs.

Question 6

Mycorrhizae

Answer 6

Mycorrhizae are fungi that grow in association with plant roots and improve the plant’s uptake of water and inorganic ions.

· The fungus colonizes the host plant’s roots. They grow in and around the roots producing a large system of threads or hyphae. · This provides the fungus with carbohydrates from the plant, such as glucose and sucrose.

· In return, the plant gains the benefits of the fungal hyphae’s greater ability to absorb water and minerals due to the large surface area of the thread like hyphae.

Question 7

Fertilisers why important

Answer 7

· Nitrates and phosphates are two very important mineral ions that are often limiting factors in plant and algal growth.

· Intensive food production results in nitrates and phosphates continually being lost from the soil as crops are harvested

· feeding livestock (e.g. cattle and sheep) which are then removed, depletes nitrates and phosphates.

· Plants are not left to decompose and so minerals are not recycled. It is therefore necessary for farmers to use fertilisers to replace the lost mineral ions and maintain crop productivity.

Question 8

Natural fertilisers

Answer 8

consist of organic plant or animal matter, containing complex organic compounds, e.g. urea, which releases mineral ions as it decays.

Question 9

Artificial fertilisers

Answer 9

are manufactured and contain concentrated mineral ions (especially NPK compounds) in an inorganic, soluble and ready-to-use form.

Question 10

Leaching

Answer 10

· excess ions e.g. nitrates and phosphates from fertiliser use, can be washed out of the soil by rain, this is leaching.

· The leached ions then find their way into streams and rivers and lakes.

· This can then cause eutrophication.

Question 11

Eutrophication

Answer 11

· An ‘algal bloom’ occurs (rapid algal growth)

· A dense layer of algae forms on the surface of the water, absorbing light and preventing it from penetrating to lower depths, so plants below the surface cannot photosynthesise and therefore die

· Increased competition between algae results in death of algae too.

· When the algae and plants die, they are decomposed by bacteria and other saprobiotic organisms, whose populations suddenly increase.

· The increase in aerobic respiration by bacteria uses up oxygen.

· The dissolved oxygen in the water becomes depleted, causing many species of invertebrates and fish to die