Nitrogen sources refers to the various natural and human-made sources of nitrogen in the environment. Nitrogen is an essential nutrient for all living organisms, including plants, animals, and microorganisms. It plays a crucial role in biological processes such as growth, metabolism, and reproduction. Some common sources of nitrogen include atmospheric nitrogen, which makes up about 78% of the Earth's atmosphere, nitrogen-fixing bacteria in the soil, nitrogen-rich organic matter such as manure and compost, and synthetic fertilizers. Research in this area focuses on understanding the different sources of nitrogen and their impacts on the environment, including nitrogen pollution, eutrophication of water bodies, and climate change. Scientists study how nitrogen cycles through the environment, how different sources of nitrogen contribute to environmental problems, and how we can manage nitrogen more sustainably to protect ecosystems and human health.