Nitrogen Fixation

Key Stage5-
Nodule experiment

Pupil worksheet

Nitrogen fixation

Plants require nitrogen to build amino acids and other molecules such as DNA and chlorophyll. However, they are unable to use atmospheric nitrogen (N2) directly. They rely on soil bacteria to fix the nitrogen into nitrates. These then dissolve into water, which the plants can absorb through their roots.

Some plants, like peas, have a symbiotic relationship with nitrogen-fixing bacteria called rhizobia, which live inside structures called root nodules. They fix atmospheric nitrogen into ammonia, which the plant can use. In return the plant supplies the bacteria with carbon-containing compounds, which they can metabolise.

Signalling between the rhizobia in the soil and the plant root cells starts pathways which promote the growth of root nodules and the development of infection threads which allow the bacteria to enter them. Once inside the cells the bacteria are known as bacteroids and will start nitrogen fixation.

Your task

Carry out an investigation to find out how the concentration of nitrate in the soil affects the growth of root nodules on peas.

Your write-up should include:

• A results table.
• An analysis of the results.
• A conclusion explaining what you discovered and scientific reasons why.

Method

1Take three 200 cm3 polystyrene or plastic cups.

2Make up enough soil/vermiculate mix to fill the three cups. Use one part soil to two parts vermiculate.

3Into each cup place one pea seed about 4 cm below the surface of the soil mix.

4Make up 1 dm3 of potassium nitrate (KNO3) solution. Your teachers will assign you a concentration (1 mM, 5 mM, 10 mM, 20 mM or 30 mM).

5Use the solution to water the soil mix. Add small amounts a bit at a time so to completely wet the soil but not waterlog it.

6Leave the peas to grow in a warm place. Water them using the potassium nitrate solution when the soil is dry. Be careful not to overwater.

7After 3 weeks of growth carefully remove the plants from the cups and rinse the roots under running tap water to remove any soil.

8Count the root nodules on each plant and calculate the mean number for one plant.

9Collaborate with the rest of the class to obtain a full set of results.