Nitrogen Cycle

The nitrogen cycle refers to the geophysical, chemical, and biological processes involved in converting atmospheric nitrogen into a form that plants can use, recycling those nitrogen compounds within ecosystems, and returning them to the atmosphere. Of all the fundamental biogeo-chemical cycles, the nitrogen cycle is the most complex as it depends on the balanced activity of several different kinds of organisms, ecosystems, and atmospheric processes. It is also the cycle most affected by human activity. Indeed, the manipulation and transportation of nitrogen compounds by humans is now a major component of this fundamental Earth system, transforming what was once a predominantly local cycle into more of a regional and even global flow of material.

Flows of nitrogen through the biosphere are important because the availability of nitrogen plays a regulatory role in the production of biomass. In regions with sufficient water and sunlight, the production of biomass is generally limited by the quantity of chemically active nitrogen available. Although Earth is encapsulated by a nitrogen-rich atmosphere, that nitrogen is locked away in strongly bonded molecules of N2. For atmospheric nitrogen to be made available to plants, those bonds must first be broken so that the nitrogen can become part of another chemical compound. In nature, nitrogen-fixing bacteria—and to a much lesser extent, lightning—perform that role. (The word fixing can be traced back to the alchemists, who used the word fixed to describe what happened when a gas was converted into a liquid or solid.)

In a mature ecological system free of agriculture, the activity of nitrogen-fixing bacteria is more than sufficient. Existing nitrogenous compounds continually get recycled as leaves, grasses, fruits, and seeds fall to the ground and decompose into ammonia (NH3) and other compounds. Then, in the soil, nitrifying bacteria (as opposed to nitrogen-fixing bacteria) convert the ammonia into nitrite (NO2-) and nitrate (NO3-), the latter being a highly soluble form of nitrogen that plants can take in through their roots. Nitrogen-fixing bacteria are only needed to make up for the small losses that occur when nitrogen-containing compounds vaporize, leach away, or get carried away by streams to wetlands, floodplains, and the sea. In soils saturated with water, other types of bacteria, known as denitrifiers, convert nitrogenous compounds back to gases, balancing the activity of nitrogen-fixers.

The emergence of stationary agriculture altered local flows of nitrogen. Transporting nitrogen-rich protein away from the same agricultural fields year after year meant that large quantities of scarce nitrogenous compounds were also being carried away. This nitrogen had to be replenished if stationary farming were to be a long-term endeavor, and much of agricultural history revolves around societies learning, by trial and error, how to accomplish this feat. In many agricultural societies, the cultivation of legumes—which work symbiotically with nitrogen-fixing bacteria—emerged as an important way to replace that nitrogen. Even then, however, the availability of nitrogen remained a limiting factor. Nitrogen-fixing bacteria could replace losses of nitrogen only so quickly, and in the long term, farmers could not produce more crops than bacterial activity allowed. Until the 19th century, societies had no choice but to live within that limit.

...