- A They are sometimes thought to actively deplete and steadily reduce the overall nitrogen content of the soil
- B They are the primary source of new biologically available nitrogen, supporting all life in the ecosystem
- C They are sometimes mistaken for obligate parasites that continuously drain nutrients from their host plants
- D They are sometimes thought to function mainly within aquatic water bodies, playing little role on dry land
Correct answer: B. They are the primary source of new biologically available nitrogen, supporting all life in the ecosystem
Explanation: Nitrogen-fixing organisms (cyanobacteria, Azotobacter, Rhizobium) introduce new fixed nitrogen into ecosystems. Without them, nitrogen would gradually be lost from soil by denitrification. In natural ecosystems without fertilizers, N2-fixers are critical for maintaining soil fertility.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.