The Phosphorus and Nitrogen Cycles

Although the effect of human activities on the carbon cycle is better known, the effect of human activities on the nitrogen cycle has been even more dramatic.

The human race's use (and abuse) of the nitrogen cycle has been one of the most beneficial for our own species for many years. Every year, humans convert 120 million tons of nitrogen from the atmosphere into reactive forms such as nitrates, mainly in the production of nitrogen-based fertilizer for crops. The widespread introduction of nitrogen fertilizers to the world's farmers was one of the driving forces of the remarkable Green Revolution of the 20th century, which increased crop yields in most regions of the globe by up to ten times.

However, human activities now remove more nitrogen from the atmosphere than all natural processes combined, and much of this nitrogen ends up as a pollutant. A particularly serious problem is nitrate pollution from agricultural runoff in ground and surface water supplies, which can not only poison humans and other living creatures drinking polluted waters, it can also significantly change freshwater and marine ecosystems. The Gulf of Mexico "Dead Zone" is caused by nitrate and phosphate pollution, primarily from agricultural runoff in the American Midwest. The fertilizers cause massive algae blooms, which consume all the oxygen in the water, leaving it too low in oxygen for anything to survive. The size of the Dead Zone varies from year to year, but it generally covers about 6000-7000 square miles and has seriously impacted fisheries in the area, as well as the health of the local ecosystem. Other Dead Zones exist in the Baltic Sea, the Black Sea, Chesapeake Bay, and some parts of the Pacific Ocean, as well as freshwater lakes and rivers around the world.

Phosphate pollution contributes to the problem as well, though to a far lesser degree. Unlike nitrogen, which is extracted from the air, phosphorus is mined.

The nitrogen cycle has already long passed its tipping point and the the human race should strive to reduce its consumption of atmospheric nitrogen to 35 million tons per year to return to sustainable levels.

One way to reduce nitrogen consumption is to increase use of nitrogen-fixing cover crops such as clovers and other legumes, and use of compost and animal manures as fertilizer.