The nitrogen cycle is the biogeochemical cycle responsible for cycling nitrogen amongst plants, animals, and the abiotic factors of their environment.
The process is a natural component of the entire Earth system. Like most biogeochemical cycles, human activities are capable of altering the natural conditions of the nitrogen cycle.
The two activities that are primarily responsible for these alterations are the use of fossil fuels and the addition of nitrogen to fertilizers.
These activities have served to increase the amount of nitrogen biologically available in the environment. The combustion of fossil fuels, including coal, oil,and natural gas, is one of the largest contributors to human disruption of the nitrogen cycle. The primary products of the combustion reaction include carbon dioxide and water vapor.
The addition of nitric oxides into the atmosphere, including NO and NO2, impacts both terrestrial and aquatic ecosystems. Once the balance of natural nitrogen in the atmosphere has been lost, the ecology of entire regions can shift. Smog and acid rain are formed by the reaction between nitric oxides and other elements, resulting in a number of negative environmental impacts. Trees will experience nutrient imbalance, forest health will begin to decline, water quality is compromised, global warming will accelerate, biodiversity is negatively impacted, and human health will suffer.
Eventually, nitric oxides settle to the soil. Here, they leach downwards or slowly drain through the soil. As the nitrogenous compounds percolate down, they strip the soil of nutrients, negatively impacting soil fertility. The added nitrogen in the soil will also serve to increase soil acidity. As the nitrogen reaches streams, lakes, and groundwater, these freshwater sources will also become more acidic.
Rivers will carry nitrogen to coastal and marine ecosystems. Here, the natural balance of the nitrogen cycle is also compromised. Microbes, plants, animals, and humans are all impacted by nitrogen pollution in the environment. In order to effectively use nitrogen in agriculture, scientists first had to figure out how to mimic the fixation of nitrogen found in nature for commercial use.
People began to understand the importance of fixed nitrogen for growing plants during the 19th century. Chilean saltpetre and ammonia collected from making coke fuel out of coal were first used in fertilizers.
Heavy agricultural areas soon had a high demand for fertilizers containing fixed nitrogen compounds, which were intended as a supplement to the naturally existing supply. During this same time frame, the demand for Chilean saltpetre for use in gunpowder spiked. It was soon realized that existing supplies of the compound were inadequate to meet future demands, spurring a search across the globe for previously undiscovered reserves.
The search for new reservoirs of nitrogenous compounds continued throughout the first decade of the 20th century.
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