The nitrogen cycle

Figure 5 shows part of the nitrogen cycle.

Use words from the box to complete the following sentences about the nitrogen cycle.

The nitrogen compounds in plants include_........

The nitrates from rotting dead plants are released into the soil by_........

180 million tonnes of nitrates were used by crops growing on farms in the UK in 2020.

Figure 6 shows the different sources of nitrates used by the crops.

Calculate the mass of nitrates from human-made fertiliser that was used by crops in 2020.

Some of the nitrates spread on fields as fertiliser get into lakes.

Name the type of pollution caused by the build-up of nitrates in lakes.

A student investigated how the concentration of nitrate affected the types of algae growing in three lakes.

Figure 7 shows microscope images of the algae in 1 mm³ of water, and the concentrations of nitrate ions in each lake.

Figure 8 shows a bar chart for the algae in lakes X, Y and Z.

Complete the bar chart for algae L and algae M in lake Z.

State two conclusions about the relationship between nitrate concentration and the algae in these lakes.

A build-up of algae on the surface of the lake can affect plants growing at the bottom of the lake.

Which statement shows how this would affect the plants at the bottom of the lake?

5 (a) A scientist decided to study the variety of living organisms in a garden.

(i) The scientist wanted to use a random sampling technique. Devise a plan the scientist could use to randomly sample the number of plant species in the garden.

(ii) The scientist also measured abiotic factors in the garden. The pH of the soil was measured using a pH meter. Describe how three other abiotic factors could be measured in the garden.

(iii) Mistletoe is a parasite that grows on some trees. Figure 6 shows a tree with mistletoe growing on it.

(Source: © NagyDodo/Shutterstock)

Figure 6

Describe the feeding relationship between the mistletoe and the tree.

(b) Nitrate fertilisers are used in the garden. Explain why nitrate fertilisers are used in gardens.

Figure 16 shows a photomicrograph of two stomata in a leaf.

The length of one guard cell in this image is 6 mm.

Convert the length of this guard cell into micrometres (µ m).

The image has been magnified 150×.

Calculate the actual size of the guard cell.

Give your answer in standard form in mm.

Explain the role of denitrifying bacteria in the nitrogen cycle.

*(c) Describe how carbon is cycled through the biotic and abiotic components of an ecosystem.

5 (a) Figure 8 shows a cross-section of a root nodule on a leguminous plant. Bacteria in the root nodule provide the leguminous plant with nitrogen compounds. The leguminous plant provides the bacteria with sugars.

(Source: © Nigel Downer / Science Photo Library)

Figure 8

(i) Which term describes the relationship between this leguminous plant and the bacteria?

(ii) The width of this root nodule is 7.5 mm. Give the width in µ m.

(b) Figure 9 shows part of the nitrogen cycle.

(i) Identify the types of microorganism involved in process X and process Y.

(ii) Explain how crop rotation increases nitrate levels in the soil.

(iii) Explain why increased nitrate levels in the soil improve crop yield.

A student investigated the decomposition of leaves from different types of tree.

Decomposers in the soil convert

The student collected and weighed some fresh leaves from a beech tree.

• The leaves were placed on top of soil in a tray.

• This was repeated for leaves from three other types of tree.

• The trays were kept in the same abiotic conditions for 40 days.

• The leaves were reweighed after 40 days.

State two abiotic factors that should be kept the same in this investigation.

Explain how this investigation could be improved to make the results more comparable.

Figure 12 shows the mean rate of decomposition for this investigation.

Which leaves would produce compost in the least time?

The same investigation was extended to include the leaves of a silver birch tree.

The starting mass of the leaves was 28.2 grams.

After 40 days the mass of the leaves was 19.7 grams.

Calculate the rate of decomposition of the leaves of the silver birch tree.

Give your answer to three significant figures and include the units.

rate of decomposition_........

Explain how the concentration of oxygen in a compost bin would affect the rate of decomposition.

(a) A student was investigating the populations of organisms in a garden.

Figure 15 shows the estimates of the number and biomass of some of the organisms in the garden.

Calculate the biomass of the population of earthworms in the garden.

Hedgehogs eat slugs and earthworms. Slug pellets were used to kill the slugs.

Explain how killing the slugs would affect the population of earthworms in this garden.

Describe a method that could be used to estimate the population of slugs in the garden.

Explain how cabbages, earthworms and squirrels contribute to the carbon cycle.

State three ways the concentration of nitrates in soil can be increased.

Figure 12 shows the global movement of carbon into or out of the atmosphere.

Calculate the net mass of carbon added to the atmosphere each year.

Joseph Priestley was a scientist who investigated how green plants and combustion affected the carbon cycle.

Figure 13 shows his first experiment.

in a bell jar
minutes

State why the candle was not burning after three minutes.

Joseph Priestley continued the investigation but placed a plant inside the bell jar as shown in Figure 14.

He observed that the candle stayed alight for two minutes more than the candle in the bell jar in the first experiment.

Explain his observation.

State two variables that would need to be controlled to compare these two experiments.

Nitrogen is cycled through the environment.

Describe the roles of bacteria in the nitrogen cycle.

(a) Plants use nitrate ions to make proteins and chlorophyll.

What effects will a low nitrate ion concentration in soils have on plants?

Which organisms convert nitrogen to nitrate ions during the nitrogen cycle?

Figure 8 shows part of a root as seen using a light microscope.

Figure 9 shows information about the two types of cell labelled in Figure 8.

Calculate the surface area to volume ratio of the root hair cell.

Explain the benefit to the plant of having root hair cells.

Algae are green plants.

Figure 10 shows the number of algae in a lake in the United Kingdom during one year.

Explain the changes in the number of algae in the lake from February to June.

(a) A student investigated the effect of nitrate ion concentration on plant growth. She placed barley seedlings in three test tubes containing different concentrations of nitrate fertiliser.

Test tube 1 contained distilled water with 1 pellet of nitrate fertiliser. Test tube 2 contained distilled water with 2 pellets of nitrate fertiliser. Test tube 3 contained distilled water with 3 pellets of nitrate fertiliser.

After 7 days, the lengths of the seedlings were measured.

Figure 13 shows an example of the apparatus used.

Describe a control for this investigation.

The nitrate fertiliser contains the chemical compound potassium nitrate. The hazard symbol on the bag of potassium nitrate fertiliser is shown in Figure 14.

Which hazard does this symbol represent?

Give a method, other than measuring the change in length, that would show the growth of the seedlings.

Figure 15 shows the results of this investigation.

Explain why there are differences in the change in the lengths of the seedlings.

Explain how nitrate ions were absorbed by the seedling in test tube 3.

Farmers use crop rotation to reduce the need to add nitrate fertilisers to the soil.

Plants such as peas and beans have a mutualistic relationship with nitrogen-fixing bacteria.

Explain why farmers use these plants in their crop rotation cycle.