Human Alteration of the Global Nitrogen Cycle: Causes and Consequences

SUMMARY Human activities are greatly increasing the amount of nitrogen cycling between the living world and the soil, water, and atmosphere. In fact, humans have already doubled the rate of nitrogen entering the land-based nitrogen cycle, and that rate is continuing to climb. This human-driven global change is having serious impacts on ecosystems around the world because nitrogen is essential to living organisms and its availability plays a crucial role in the organization and functioning of the world’s ecosystems. In many ecosystems on land and sea, the supply of nitrogen is a key factor controlling the nature and diversity of plant life, the population dynamics of both grazing animals and their predators, and vital ecological processes such as plant productivity and the cycling of carbon and soil minerals. This is true not only in wild or unmanaged systems but in most croplands and forestry plantations as well. Excessive nitrogen additions can pollute ecosystems and alter both their ecological functioning and the living communities they support. Most of the human activities responsible for the increase in global nitrogen are local in scale, from the production and use of nitrogen fertilizers to the burning of fossil fuels in automobiles, power generation plants, and industries. However, human activities have not only increased the supply but enhanced the global movement of various forms of nitrogen through air and water. Because of this increased mobility, excess nitrogen from human activities has serious and long-term environmental consequences for large regions of the Earth. The impacts of human domination of the nitrogen cycle that we have identified with certainty include: • Increased global concentrations of nitrous oxide (N 2 O), a potent greenhouse gas, in the atmosphere as well as increased regional concentrations of other oxides of nitrogen (including nitric oxide, NO) that drive the formation of photochemical smog; • Losses of soil nutrients such as calcium and potassium that are essential for long-term soil fertility; • Substantial acidification of soils and of the waters of streams and lakes in several regions; • Greatly increased transport of nitrogen by rivers into estuaries and coastal waters where it is a major pollutant. We are also confident that human alterations of the nitrogen cycle have: • Accelerated losses of biological diversity, especially among plants adapted to low-nitrogen soils, and subsequently, the animals and microbes that depend on these plants; • Caused changes in the plant and animal life and …