Grouppe Kurosawa: Organic Gardening - Nitrogen Cycle (print)

Nitrogen Cycle

Plants are no different from any other organism. They need a source of carbon, nitrogen, sulfur, and phosphorus in order to survive. Of course, they need a host of trace minerals also, but they are not usually in short supply in the soil. Plants obtain their necessary carbon from the CO2 in the air. During the process of photosynthesis, they convert the carbon in CO2 into sugars such as glucose. Glucose is either metabolized directly in the process of respiration or it is converted into other forms such as sucrose, cellulose, starch, vitamins and other carbon-based molecules. Since phosphorus and sulfur are not a component of most carbon-based molecules, the availability of nitrogen becomes the limiting factor in the growth of most plants and microorganisms.

Nitrogen comes from many sources. These sources include atmospheric nitrogen or N2, mineral nitrogen that is a product of volcanic action, and nitrogen that is released from decaying plants and animals.

Certain bacteria can “fix” N2 directly from the air and convert it into NH2, a form that plants can use. These bacteria, Rhizobium, live in nodules that are associated with the roots on legumes (beans, pleas, alfalfa, vetch, soybeans). Since these plants obtain their necessary nitrogen from the air, they do not need chemical nitrate or ammonium fertilizers. Scientists have been trying for 40 years to clone the nitrogen fixing genes from Rhizobium into plants in order to eliminate their need for chemical fertilizers. To our knowledge, no one has succeeded. We doubt that chemical fertilizer companies are spending any money on this research since it would put them out of business.

When plants, animals, fish, birds, and microorganisms die, their bodies are broken down and recycled by other microorganisms in the soil or ocean. One of the most important steps in their recycling process is the removal of nitrogen from cell proteins and its conversion into a form that can be reused by plants and microorganisms. This process is called ammonification and it refers to the conversion of NH2 from cell proteins into NH3 or ammonia (the famous dead fish smell). Ammonia is a gas at room temperature. In the soil, it mixes with water and forms NH4+ or ammonium ion. This ion is directly taken up by plants and is eventually converted into NH2 and incorporated into newly formed molecules. Ammonia can also be directly converted into nitrates by various soil bacteria. First, NH3 is converted into NO2 (nitrite) by bacteria such as Nitrosomonas. Nitrites are not very soluble in water and are further converted into nitrates by bacteria such as Nitrobacter. Nitrates are VERY soluble in water and are responsible for much of the fertilizer contamination of the water table. Nitrates can be taken up by plants and converted into NH2 for incorporation into different molecules. Microorganisms can also utilize nitrates and ammonium ion for their own metabolism. When humans, animals, birds or fish eat plants and/or other animals, they convert the plant and animal nitrogen into their own. When plants and animals die, bacteria and fungi start the nitrogen cycle all over again. Without microorganisms in the soil and ocean, all life on earth would cease.