Human activities have changed the nitrogen cycle in some ecosystems by adding excessive amounts of nitrogen to the soil. Although nitrogen is a limiting factor to the growth of animals and plants in some ecosystems, humans have severely increased the amount of available nitrogen due to agricultural and industrial activities. Too much nitrogen, typically due to the addition of fertilizers, is known as nutrient pollution.

You learned in week 2 that nutrient pollution causes eutrophication. Eutrophication leads to disastrous results for aquatic ecosystems. For example, when nitrogen-rich sewage and fertilizers pour into lakes, ponds, streams, and rivers, the result is an increased growth in algae (known as an algal bloom). When the algae eventually die, they are decomposed by bacteria, a process that requires a lot of oxygen. Following an algal bloom, decomposing bacteria in aquatic ecosystems become so abundant and oxygen levels so depleted that fish and other aquatic life may die.

In the years since scientists discovered this connection, cities and farmers have taken measures to control the flow of nitrogen into ecosystems, so that nitrogen is available in the right quantities. Organic agricultural techniques such as terracing fields can reduce the amount of eroded soil that enters aquatic ecosystems. Other farmers have switched from using man-made fertilizers to natural forms (e.g. cow manure) to fertilize fields. Planting nitrogen-fixing crops, such as clover, also reduces the amount of nitrogen that could potentially wash into nearby water. The following video describes some of the techniques used by organic farmers to reduce the impact of nutrient pollution on the environment.



Source: Interactive NOVA: "Earth"