How do humans alter the nitrogen cycle?

Humans contribute by using nitrogen-containing fertilizers in agriculture. Under conditions in which agricultural plants have access to as much water as they require, their productivity is usually constrained by the rate at which they can obtain nitrogen in available forms, particularly nitrate, and sometimes ammonium. Under such conditions, farmers attempt to increase the availability of these nutrients, usually by applying fertilizers. 

Humans also influence the nitrogen cycle by dumping sewage and other types of organic matter into water-bodies. There is a great deal of environmental damage associated with these practices, including lowered dissolved oxygen levels associated with microbial oxidation of the organic matter, and the presence of fecal pathogens and parasites. The accidental fertilization of waterbodies with large amounts of nitrogen contributes greatly to eutrophication. 

Humans also affect the nitrogen cycle through the emissions of large quantities of NOx gases to the atmosphere. The most important sources of emission are automobiles, power plants, home furnaces, and factories. The emitted NOx is an important air pollutant, because it is critical in the photochemical oxidative reactions by which toxic ozone is formed, and because the NOx is an important source of nitrate in acidic precipitation.