Although there's plenty of nitrogen in the air, it's not in a form plants can use. They can only absorb nitrogen in the form of ammonium, nitrite, and nitrate. One way plants have access to these forms of nitrogens is decomposition of organic matter by soil organisms. Another source of usable nitrogen are mutualistic relationships between plants and bacteria. By providing nitrogen-fixing bacteria a place to live and feeding them with carbohydrates, plants get nitrogen in the form of ammonium. Plants that can do this include most legumes which form relationships with Rhizobium bacteria, and other plants like alder, bayberry, California lilac, and Russian olive that support Frankia bacteria on their roots.
There are also free-living nitrogen-fixing bacteria, as well as cyanobacteria that grow on the surface of plants or the soil, that add small amounts of plant-usable nitrogen to the soil. Lightning also generates usable nitrogen compounds.
Chemical fertilizer is a major source of plant-usable nitrogen. But other human activities, like the burning of fossil fuels and livestock operations, inadvertently produce widespread, nitrogen-enriched rainfall.
