CAIE IGCSE Biology 19.3.3 Roles of microorganisms

Practise matching microorganisms to nitrogen-cycle stages, especially bacterial roles shown on pathway diagrams.

Exam points

choose which labelled nitrogen-cycle processes are carried out by bacteria
distinguish decomposition, nitrification, nitrogen fixation and denitrification by role

[Maximum number: 1]

A student investigated the effect of high temperature on the production of nitrate ions in soil.
Two samples of soil were taken. One sample was heated to $100^{\circ} \mathrm{C}$ .
All the nitrate ions were completely removed from both soil samples.
Ammonium ions were then added to both soil samples.
After two weeks, both soil samples were tested for the presence of nitrate ions.
The results are shown.

Table for Question 19.3.3—Roles of microorganisms question 1 — CAIE IGCSE Biology

Which statement explains the results?

Heating the soil broke down the nitrate ions.

Heating the soil increased the activity of denitrifying bacteria.

Heating the soil killed nitrifying bacteria.

Heating the soil killed nitrogen-fixing bacteria.

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19.3.3—Roles of microorganisms question 1