The Bacteria That Lives in the Soil & Fixes Nitrogen

Nitrogen Fixation

• Bacteria "fix" nitrogen by chemically bonding gaseous nitrogen from the air with hydrogen, producing ammonia. This process is accomplished by an enzyme produced by the bacteria, nitrogenase, which contains two proteins that react with nitrogen and result in the formation of ammonia. This chemical reaction must occur in the absence of oxygen, and nitrogen-fixing bacteria have evolved various mechanisms for protecting the process from exposure to oxygen.

Rhizobium

• Some species of nitrogen-fixing bacteria, the most important of which is Rhizobium, live in a symbiotic relationship with host plants. The bacteria infect the plant by attaching to tiny hairs on the growing root, and the plant responds by producing chemicals called flavonoids. The presence of flavonoids causes the bacteria to produce substances that accumulate into a nodule, inside which the fixation process occurs. The plant provides sugars to the bacteria and is nourished by fixed nitrogen generated by the bacteria.

Infected Plants

• Rhizobium bacteria live in the roots of legumes, such as peas, beans and clover. Other legumes, such as soybeans and chickpeas, are infected by smaller bacteria called Bradyrhizobia. Bacteria of the Frankia genus infect some woody trees and shrubs, and nitrogen-fixing cyanobacteria live in the leaves of water ferns and cycads. These infections do not harm the plant, and, in the case of Rhizobium infections, the bacteria is surrounded by a plant-produced barrier and is never allowed access to the root itself.

Agricultural Benefits

• Nitrogen fixation by bacteria is a significant source of a vital nutrient for plants, and, in the case of legumes, nitrogen-fixing bacteria are the plant's primary source of nitrogen. To ensure a healthy crop, legume growers inoculate their plants, usually mixing the bacteria with the seed at the time of planting, thereby increasing the chances that a healthy number of ammonia-producing root nodules are formed. Ammonia produced by the bacteria can improve the overall nutritional content of the soil as well, and sometimes cover crops such as clover are planted in fields during the off-season so that other crops can benefit from increased soil nitrogen levels in subsequent growing seasons.