The nitrogen cycle

The diagram shows the nitrogen cycle with some processes labelled.

Explain why plants need nitrate ions.

Name the processes V, X and Y.

Scientists measure the dissolved nitrate levels in a river that passes through farmland. They also measure the nitrogen compounds being used on the farmland either as fertiliser or as manure from livestock. The graph shows how the nitrate levels in the river and the nitrogen compounds applied to the farmland changed from 1950 to 2005.

(Source: Graph: Trends in Nitrate Levels in Maumee River | U.S. Geological Survey (usgs.gov))

Comment on the relationship between nitrate levels in the river and the nitrogen compounds applied to the farmland.

Describe the changes within the river that would have occurred between 1950 and 2000.

Read the passage below. Use the information in the passage and your own knowledge to answer the questions that follow. When the Oceans Glow In some areas of the world the oceans around the coast sometimes glow with a blue light. The photograph shows an area of coast that is glowing. This blue light is produced by the presence of thousands of microscopic, living organisms called dinoflagellates. These dinoflagellates are protoctists. The production of light by living organisms is called bioluminescence, a process that has evolved many times in different species of organism.

(Source: © AMIRREZA KAMKAR/SCIENCE PHOTO LIBRARY) To generate light, dinoflagellates use special proteins and the ATP produced within their cells. Many species of dinoflagellate contain chlorophyll and are able to photosynthesize. The appearance of glowing dinoflagellates in the sea used to be a rare event but this now occurs much more frequently. Many of the events occur in the sea around river estuaries and scientists think that intensive farming and deforestation could be to blame. Due to overpopulation of dinoflagellates in these areas, other species of animal are often harmed. After a series of glowing events, large numbers of dinoflagellates die causing oxygen levels in the water to decrease. People have often wondered why dinoflagellates glow. They only glow in areas where the water moves around, such as when waves hit a beach. Scientists now think that the production of light is a type of warning to stop predators eating the dinoflagellates. If an animal eats dinoflagellates, the dinoflagellates in the area glow making the animal obvious to its own predators. To test this, scientists placed dinoflagellates into a tank along with 15 copepods, which are predators of dinoflagellates. When the dinoflagellates glowed, the copepods ate 1200 dinoflagellates in two hours. When the dinoflagellates did not glow, the copepods ate 2100 dinoflagellates in two hours. Some scientists think that we could make use of the dinoflagellates to provide sustainable street lighting. Tanks of dinoflagellates could be placed on top of lamp posts. The dinoflagellates would photosynthesise during the day when it is light. A stirrer powered by a small battery would then move them at night so that they would glow. These sustainable lamps could be carbon neutral and help to reduce pollution.

Which of these processes produces ATP? (line 6)

Explain why intensive farming and deforestation would cause an increase in populations of dinoflagellates. (lines 9 to 11)

Explain why oxygen levels decrease after a series of glowing events. (lines 10 to 13)

Which type of bacteria converts ammonia into nitrates?

Each of the 15 copepods ate glowing dinoflagellates at a mean rate of 40 dinoflagellates per hour. Calculate the mean rate at which each copepod ate dinoflagellates that were not glowing. (lines 18 to 21)

mean rate =_........ dinoflagellates per hour

Explain how natural selection could have resulted in the evolution of dinoflagellates that glow. (lines 15 to 18)

Explain why using dinoflagellates for street lighting would help to reduce pollution. (lines 22 to 26)

The diagram shows the nitrogen cycle. Some of the stages have been labelled.

Which stage shows nitrogen fixation?

Which stage shows nitrification?

Which stage shows denitrification?

Farmers sometimes add chemical fertiliser to the soil.

Describe how pollution by fertiliser can affect aquatic ecosystems.

Give an alternative to chemical fertiliser that a farmer could use.

The graph shows the change in area of rainforest on the Earth from 1990 to 2015.

Determine the mean rate of decrease of rainforest area, in km² per year, from 1990 to 2015.

mean rate of decrease =_........ km² per year

Loss of rainforests causes an increase in atmospheric carbon dioxide gas.

Explain the negative effects of an increase in atmospheric carbon dioxide gas.

Scientists investigated the effect of deforestation on a river. The river flows through a forest, part of which has been deforested.

The graphs show the changes in nitrate concentration and the biodiversity of animals in the river as it flows through the forest.

Describe the role of microorganisms in the conversion of organic waste into nitrate ions in the river.

The number of different animal species living in the river was recorded as a measure of the biodiversity of animals.

Give a reason why this may not be a complete measure of biodiversity.

Explain the change in the biodiversity of animals along the river.

Use information from the graphs in your answer.

A student uses this method to investigate the nitrogen cycle.

• take two samples of soil, each of mass 100 g

• sterilise one sample of soil by heating at 100 °C for one hour

• place the sterilised and unsterilised samples into separate filter funnels

• pour 25 cm³ of water through each soil sample and collect the filtrate in a test tube

• test each filtrate for nitrates

• pour water through each soil sample for 5 minutes

• pour another 25 cm³ of water through each soil sample and collect the filtrate in a test tube

• test each filtrate for nitrates

• add 1 cm³ of a solution of ammonium salts to each soil and leave for three days

• pour 25 cm³ of water through each soil sample again and collect the filtrate in a test tube

• test each filtrate for nitrates

The diagram shows the student’s apparatus.

The table shows the student’s results.

Give the independent variable in the investigation.

Suggest why the student poured water through the soil samples for five minutes before adding the ammonium salts.

Comment on the results of the nitrate tests on the two soil samples three days after adding ammonium salts.

Fish farming is often used to produce protein rich food.

Selective breeding is often used to produce fish that grow rapidly and do not waste much food. Explain how selective breeding can be used to produce fish that grow rapidly.

Fish farming systems can often release ammonia into the water. The ammonia is converted into nitrates. Describe how ammonia is converted into nitrates.

Multi‑trophic level aquaculture is a method of fish farming that has been developed to reduce environmental pollution and increase profits. The diagram shows a multi‑trophic level aquaculture system.

Explain how the multi‑trophic level aquaculture system reduces environmental pollution, and increases the profits of fish farming. Use information from the diagram and your own knowledge to support your answer.

The diagram shows some parts of the nitrogen cycle.

Name the processes labelled A, B and C.

The mass of nitrogen that moves within the nitrogen cycle has been estimated and some of the masses are given in the table.

Some of these processes remove nitrogen from the atmosphere. Calculate the total mass of nitrogen removed from the atmosphere by these processes. Give your answer in standard form.

Calculate the percentage of the nitrogen released into the atmosphere that comes from burning of biomass.

Explain how burning biomass returns nitrogen to the atmosphere.

Explain the effect of nitrous oxide on global warming.

The diagram shows the nitrogen cycle with four processes labelled A, B, C and D.

(Source: https://www.shutterstock.com/image-vector/diagram-nitrogen-cycle-process-by-which-1400843660)

Which process is nitrogen fixation?

Which process is decomposition?

Which process is nitrification?

Name a type of organism that carries out process C.

A student does an investigation to determine if nitrate ions are required for plant growth.

He uses this method.

Step 1 Use a measuring cylinder to add 10 cm3³ of the control solution containing all the minerals a plant requires to a test tube Step 2 Cover the top of the tube with foil Step 3 Make a small hole in the foil Step 4 P ush the root of a germinated bean seedling through the hole so it is in the solution Step 5 Rinse the measuring cylinder with distilled water Step 6 R epeat Steps 1 to 4 using a test solution containing all the mineral a plant requires apart from nitrate ions Step 7 Wrap both tubes in black paper Step 8 Place both tubes in a test tube rack in bright sunlight

Explain why the control solution contains all the mineral ions the plant requires but the test solution contains all the mineral ions the plant requires except nitrate.

State the purpose of Step 5.

Explain the purpose of Step 7.

Explain the measurements that the student could make to determine if nitrate ions are required for plant growth.