Human alteration of the nitrogen cycle and its environmental consequences

Early in the 20th century, a German scientist name Fritz Haber figured out how to alter the nitrogen cycle by fixing nitrogen at high temperatures and pressures, creating fertilizers that could be added directly to the soil. This technology has spread rapidly over the past century, and, along with the advent of new crop varieties, the use of synthetic fertilizers has led to an enormous boom in agricultural productivity. This agricultural productivity has helped us to feed a rapidly growing world population, but the increase in nitrogen fixation has had some negative consequences as well. While the consequences are perhaps not as obvious as an increase in global temperatures or a hole in the ozone layer, they are just as serious and potentially harmful to humans and other organisms.

Not all of the nitrogen fertilizer applied to agricultural fields stays to nourish crops. Some is washed off of agricultural fields by rain or irrigation water where it leaches into surface or groundwater and can accumulate. In groundwater that is used as a drinking water source, excess nitrogen can lead to cancer in humans and respiratory distress in infants. The U.S. Environmental Protection Agency has established a standard for nitrogen in drinking water of 10 mg per liter (mg/L). Unfortunately, many systems (particularly in agricultural areas) already exceed this level. By comparison, nitrate levels in waters that have not been altered by human activity are rarely greater than 1 mg/L.

Nitrates and ammonia present in surface water and soil can also enter the atmosphere as the smog-component nitric oxide (NO) and the greenhouse gas nitrous oxide (N₂O). Eventually, this atmospheric nitrogen can be blown into nitrogen-sensitive environments, causing long-term changes. For example, nitrogen oxides are responsible for a significant portion of the acidity in acid rain, which has been blamed for forest death and decline in parts of Europe and the Northeast U.S. Increases in atmospheric nitrogen deposition have also been blamed for changes in species diversity and community composition, as well as ecosystem function in some forest and grassland ecosystems. For example, on nitrogen-poor soils of northern Californian grasslands, plant communities have been historically limited to native species that can survive without a lot of nitrogen. There is now some evidence that elevated levels of atmospheric nitrogen input from nearby industrial and agricultural development have paved the way for invasion by non-native species of plants. As noted earlier, NO is also a major factor in the formation of smog, which is known to cause respiratory illnesses like asthma in both children and adults.

Currently, much research is devoted to understanding the effects of nitrogen enrichment in the air, groundwater, and surface water. Scientists are also exploring alternative agricultural practices that will sustain high productivity while decreasing the negative impacts caused by fertilizer use. These studies not only help us quantify how humans have altered the natural wold, but increase our understanding of the processes involved in the nitrogen cycle as a whole.


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