(Click on title of this post for link to the entire Nitrogen article below.)
Another way nitrogen enters the cycle is as inorganic nitrogen from the atmosphere and factories. The concern with these forms is that the incremental amount of nitrates they add to the nitrogen cycle may threaten groundwater.
- Rain storms contribute atmospheric nitrogen through rain drops that reach the soil.
- Legumes, such as soybeans, alfalfa and clovers, are plants that can convert atmospheric nitrogen into plant-usable nitrogen.
- Factories that produce nitrogen fertilizers add nitrogen to the soil when farmers and gardeners "feed" their crops.
- Nitrogen in sewage sludge from municipal waste plants can be used to fertilize farm fields.
For the most part, the nitrogen cycle is soil based. Nitrogen is lost from the cycle in four ways:
Bacteria change nitrate in the soil to atmospheric nitrogen, which joins the atmosphere.
Turns urea fertilizers and manures on the soil surface into gases that also join the atmosphere.
Together, these first two processes account for most of the nitrogen lost to the cycle — a concern for soil fertility.
Carries the nitrogen in fertilizers and manure and the nitrogen in the soil into our rivers and streams — a concern for water quality.
Carries nitrates soo deep into the soil that plants can no longer use them, producing a dual concern — for lost fertility and for water quality, as nitrates enter the groundwater and the wells that provide our drinking water.
The largest single source of nitrogen is the atmosphere. It is made up of 78 percent of this colorless, odorless, nontoxic gas. However, plants are unable to use nitrogen as it exists in the atmosphere. Nitrogen from the air (N2) enters the nitrogen cycle through several unique types of microorganisms that can convert N2 gas to inorganic forms usable by plants. Some of these microorganisms live in the soil, while others live in nodules of roots of certain plants.
Nitrogen also can enter the cycle from other sources besides the air, manure and decaying plant materials. Nitrogen also can enter the cycle from the application of commercial nitrogen fertilizers.
Nitrogen can be lost from the cycle. It can be lost to the atmosphere, removed by harvesting crops or lost to surface water or groundwater. However it is lost, nitrogen can enter the cycle again through one of the processes discussed above or through other processes. These additional pathways of gains and losses to the nitrogen cycle are illustrated in Figure 2.
A more comprehensive look at the nitrogen cycle.