Humans affect the nitrogen cycle through activities that increase the amount of nitrogen that is biologically available in an ecosystem. The major culprits are fossil fuel combustion and the application of nitrogen-based fertilizer. Fossil fuel combustion releases nitric oxides and combines with other elements in the air to form smog and acid rain.
The US National Library of Medicine reprinted a study by Scott Fields that concludes humans inject nitrogen compounds into the air in ever increasing quantities, causing serious impacts on biodiversity, human health, water quality, global warming and also the rate of population growth in developing nations.
Most scientists agree that humans disrupt the nitrogen cycle by altering the amount of nitrogen stored in the biosphere. Burning fossil fuels results in a change in carbon storage, thus affecting the nitrogen cycle and other vital processes. It adds nitrogen to terrestrial ecosystems and leads to nutrient imbalance in trees, a decline in biodiversity, and changes in the health of forests.
Moreover, agricultural systems use fertilizers to improve plant production. Unused nitrogen, typically in the form of nitrate, tends to enter rivers and streams and eventually reach people’s drinking water. It also enters coastal marine systems near the shore and causes anoxia, which means no oxygen, and hypoxia, which means low oxygen. It alters biodiversity, degrades habitat and changes the food-web structure.
An increase in the availability of nitrogen usually limits the productivity of both terrestrial and aquatic systems. Due to human activities, industrial nitrogen fixation has exponentially increased over the years.