OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which process adds nitrogen to the soil?

1. Photosynthesis

2. Respiration of plants

3. Nitrogen fixation by lightning and bacteria

4. Evaporation of water

The correct answer is: Nitrogen fixation by lightning and bacteria

The addition of nitrogen to the soil is primarily facilitated through the process of nitrogen fixation. This process can occur in two main ways: through lightning strikes that convert atmospheric nitrogen into forms usable by plants, and through bacteria, such as those found in the root nodules of legumes, which convert nitrogen gas from the atmosphere into ammonium and nitrate forms. These compounds are essential nutrients for plants and contribute to soil fertility. In contrast to nitrogen fixation, photosynthesis involves the conversion of carbon dioxide and sunlight into glucose and oxygen, while respiration refers to the process by which plants and animals convert glucose and oxygen into energy, thereby releasing carbon dioxide and water. Evaporation pertains to the transition of water from liquid to gas, which does not contribute to nitrogen levels in the soil. Hence, the correct choice highlights the critical role that specific natural processes play in maintaining soil nitrogen content essential for plant growth.