Plants that encourage the proliferation of nitrogen-fixing bacteria are d. legumes. Legumes are plants that are in a symbiotic relationship with bacteria that live in their root system. Plants cannot directly use atmospheric nitrogen. Thanks to the presence nitrogen-fixing bacteria in the root, legumes use atmospheric nitrogen indirectly because bacteria convert it into ammonia and ammonium available to plants. After the death of legumes, nitrogen from their remainings is available to the other plants.