Nitrogen Fixation

Plant growth is directly dependent upon an adequate supply of fixed nitrogen. Water and fixed nitrogen are the most common limiting factors for plant growth.There is an abundance of nitrogen in the atmosphere, but it is in the form of the relatively inert gas, N2, and this must be converted chemically or biologically into a more active form before plants can utilize it. Chemical fixation most commonly is accomplished by the Haber process, which reduces N2 to ammonia catalytically at high pressure and high temperature. More recent experiments indicate that N2 in a complex with tungsten can react with H2 in a ruthenium complex to yield ammonia under mild conditions, but this is not a commercialized process. When electrical current is inexpensive, it is practical to fix N2 by oxidation to nitrate in an electrical discharge. Chemical nitrogen fixation constitutes amajor industry, as current fixation worldwide produces about 85 million metric tonnes of fixed nitrogen a year. Nitrogen fixed as ammoniamay be added as anhydrous ammonia directly to soil as a fertilizer, or it can be converted to an ammonium salt, urea or nitrate before use. As fixation is an energydemanding process, chemical fixation uses large quantities of fossil fuels (commonly natural gas) with accompanying production of carbon dioxide. In contrast, biological fixation is innocuous ecologically. Although it is difficult to determine the amount of nitrogen fixed biologically, estimates suggest that perhaps 150 million metric tonnes are fixed worldwide annually, or about twice the amount fixed by the chemical industry. Blue-green algae (Cyanobacteria spp.) are active fixers in the oceans, but they are dispersed, so the amount of nitrogen they fix can only be estimated roughly.