Nitrogen-cycling bacteria are microorganisms that play a crucial role in the nitrogen cycle, a natural process that converts nitrogen in the atmosphere into forms usable by living organisms. These bacteria help in various stages, including nitrogen fixation, where atmospheric nitrogen is converted into ammonia, and nitrification, where ammonia is transformed into nitrites and then nitrates. These bacteria are essential for soil health and fertility, as they help plants access nitrogen, a vital nutrient for growth. Key groups of nitrogen-cycling bacteria include Rhizobium, which forms symbiotic relationships with legumes, and Nitrosomonas and Nitrobacter, which are involved in nitrification.