Certified Crop Advisor Practice Exam

In which organism does symbiotic nitrogen fixation primarily occur?

1. Grasses

2. Legumes

3. Fungi

4. Stone fruits

The correct answer is: Legumes

Symbiotic nitrogen fixation primarily occurs in legumes due to their unique relationship with specific bacteria, particularly those in the genus Rhizobium. These bacteria colonize the root nodules of the legume plants, where they convert atmospheric nitrogen (N2) into ammonia (NH3), a form of nitrogen that plants can use for growth. This process is beneficial for both the bacteria, which receive carbohydrates and a suitable environment from the legumes, and the legumes themselves, which gain access to a vital nutrient that enhances their growth and productivity. Legumes are especially important in agricultural systems because they can improve soil fertility through this nitrogen-fixing process, allowing for more sustainable farming practices. The other organisms listed—grasses, fungi, and stone fruits—are not primarily associated with this specific form of nitrogen fixation. While some grasses can host nitrogen-fixing bacteria, they do not do so symbiotically in the same way that legumes do. Fungi can form mutualistic relationships with plants (mycorrhizae), but they do not perform nitrogen fixation. Stone fruits, being flowering plants, do not have a significant association with nitrogen-fixing bacteria either. Thus, legumes stand out as the primary organism benefiting from symbiotic nitrogen fixation.