Some bacteria live in the roots of plants like soybeans and peas.

Bacteria growing on plant roots.

What is the role of these bacteria in the nitrogen cycle?

Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N2) to ammonia (NH3) in a process called nitrogen fixation.