Since the Industrial Revolution, human sources of nitrous oxide emissions have grown. Activities such as agriculture, fossil fuel combustion and industrial processes are the primary cause of the increased nitrous oxide concentrations in the atmosphere. Together these sources are responsible for most of all human nitrogen oxide emissions. Other sources include biomass burning, atmospheric deposition and human sewage.
Fossil fuel combustion and industrial processes are an important source of nitrous oxide emissions. These two combined are responsible for 10% of human emissions which equals 700,000 tons of nitrous oxide annually. Nitrous oxide is a by-product of fuel combustion in mobile and stationary sources. When any fossil fuel is burnt part of the nitrogen that is in the fuel and surrounding air gets oxidized creating nitrogen oxide emissions. The majority of stationary emissions come from coal fired power plants. As for mobile emissions, almost all of it comes from cars and trucks that are used to transport people and goods. Industrial processes also cause nitrous oxide emissions. The two main industrial sources are the production of nitric and adipic acid. Nitric acid is an important ingredient for synthetic fertilizers, while adipic acid is primarily used for making synthetic fibers. For both of these acids, oxidization of nitrogen compounds during the production process creates nitrous oxide. In transportation sector, nitrous oxide is emitted when transportation fuels are burned. Motor vehicles, including passenger cars and trucks, are the primary source of nitrogen emissions from transportation. The amount of nitrogen emitted from transportation depends on the type of fuel and vehicle technology, maintenance, and operating practices.
Another source of nitrous oxide emissions is from agriculture. Agriculture creates both direct and indirect emissions. Direct emissions come from fertilized agricultural soils and livestock manure. While indirect emissions come from runoff and leaching of fertilizers. Agriculture creates 4.5 million tons of nitrous oxide annually. Because of the inefficiencies of nitrogen uptake by plants and animals, only about 10 to 15% of reactive nitrogen ever enters a human mouth as food. The rest is lost to the environment.