Nitrogen Fixation

Abstract

About 80% of the Earth's atmosphere is made up of nitrogen gas (N 2 ), but nitrogen is most often the limiting nutrient for agricultural productivity, because neither plants nor animals can use it directly. It must first be fixed into either ammonium or nitrate. Only the simplest life forms, bacteria and archaea, can produce ammonium from N 2 using the enzyme called nitrogenase. N 2 is also fixed by nonbiological processes, which contribute equally to the total annual fixation rate. In fact, without commercial fertilizers, the human population could not be supported. This article first describes the major industrial processes and their shortcomings, emphasizing the dominant Haber‐Bosch process. Then, an overview of the various biological nitrogen‐fixing systems, both free‐living and symbiotic organisms, focusing on the agriculturally important legume– Rhizobium symbiosis is provided. Nitrogenase is detailed and analyzed next with sections on its evolution, biosynthesis, composition and structure, regulation, and catalytic mechanism, the last of which is now beginning to be understood in some detail. The penultimate section describes attempts to duplicate either the reactivity or the structure of nitrogenase in chemical systems, some of which have achieved limited catalytic capability. Finally, the last section attempts to highlight both the important strides made and the problems that remain. These include limiting losses of applied fertilizer to ground water and atmosphere; careful matching of inoculant with cultivar; reducing fossil fuel energy inputs to fertilizer production; manipulating plant colonization and symbiosis development to produce new pairings; and simplifying the biological process for transfer to other organisms.