Ecology in Action

Humans have profoundly changed nutrient cycles on a global, regional, and local level. Agricultural runoff carrying heavy loads of nitrogen and phosphorus compounds caused eutrophication of the Black Sea. This led to a series of events that culminated in the annual formation of a dead zone within the Black Sea, and the consequent loss of biological diversity of several trophic levels. The nitrogen cycle depends heavily on the activities of microorganisms to fix nitrogen, and to transform nitrogen in the processes of nitrification, ammonification, denitrification, and anammox. Technological advances such as the Haber–Bosch process have vastly increased the amount of reactive nitrogen entering ecosystems, leading to increases in agricultural production, but also polluting many aquatic systems. The phosphorus cycle is similar to the nitrogen cycle, in that globally there are vast stores of phosphorus compounds, but most of it is inaccessible to organisms. In contrast to the nitrogen cycle, there is only a small atmospheric component to the phosphorus cycle; most phosphorus becomes available through weathering of rocks. Both nutrient cycles are similar in one very important way; nitrogen and phosphorus are recycled many times between organisms and the environment before exiting an ecosystem.