The Nitrogen N Cycle

The N cycle refers to the circulation of N compounds through the earth's atmosphere, hydrosphere, biosphere and pedosphere. At various points in this cycle, chemically reactive N compounds influence GHG balances. Reactive N is added to soils mainly in the forms of fertilizer and manure and as a result of biological N fixation, although the use of biosolids and atmospheric N deposition from fossil fuel combustion may also be significant in certain areas.

To describe the N cycle simply, N moves from the soil to the plant, and back from the plant to the soil, often with animals or humans as intermediaries. In reality, the situation is more complex because N compounds undergo a number of transformations in the soil (mineralization, immobilization, fixation, nitrification and denitrification). These compounds are exchanged between the soil and the atmosphere (through volatilization, denitrification, biological N fixation, atmospheric deposition) and between the soil and the hydrosphere (through leaching, erosion/runoff, drainage, irrigation). These transformations and fluxes constitute the soil N cycle.

In natural ecosystems, this cycle is more or less closed, with N inputs balancing N losses. However, the relatively small amount of available N in most natural ecosystems limits biomass production.

In agricultural systems, the N cycle is significantly altered by the export of substantial amounts of N in harvested products to support current population levels. Consequently, application of fertilizers containing N and other crop nutrients is essential to balance inputs and outputs, to maintain or even improve soil fertility, to increase productivity on current crop- and grassland and, in turn, to preserve forests, natural ecosystems and wild habitats from conversion to farming.

Appropriate N inputs ensure and enhance soil fertility, sustainable agriculture, food security (enough calories) and nutrition security (a balanced diet containing all essential nutrients, including protein). When improperly managed, N inputs can be associated with a number of adverse effects on both the environment and human health. With regard to climate change, insufficient reactive N in the agro-ecosystem leads to low crop yields, fewer crop residues and to depletion of soil organic matter, an important carbon sink. In addition, low-yielding agriculture leads to more forest and other land with high C sequestration potential being converted into arable or grassland, which reduces the level of stored carbon. On the other hand, excess amounts of reactive N may increase the atmospheric concentration of N2O, a potent greenhouse gas.

With regard to climate change, insufficient reactive N in the agro-ecosystem leads to low crop yields, fewer crop residues and to depletion of soil organic matter, an important carbon sink. In addition, low-yielding agriculture leads to more forest and other land with high C sequestration potential being converted into arable or grassland, which reduces the level of stored carbon. On the other hand, excess amounts of reactive N may increase the atmospheric concentration of N2O, a potent greenhouse gas.

Many interactions occur between the C and N cycles because of the key role of these two components in the synthesis of organic molecules. Enhancing the availability of reactive N through industrial or biological N fixation results in increased C sequestration in plant organic matter, provided other factors, such as water supply, do not limit photosynthesis.

The soil nitrogen cycle (Adapted from Hofman and Van Cleemput, 2004)