Biological Dinitrogen Fixation in Agriculture.

In the quarter century since the publication of a similar chapter on dinitrogen fixation by Havelka, Boyle, and Hardy in Nitrogen in Agricultural Soils, tremendous advances have been made in our understanding of the bacteria, host plants, and their symbiosis, and in ways to manage this source of N in agriculture. The current chapter summarizes the findings of more than 600 recent references about this key biological process. Successful invasion of the host by rhizobia, revealed by new techniques in microscopy, requires excellent communication between the partners. The means of communication include several gene products, and their source genes and resulting biochemical pathways that are active during infection, nodulation, and dinitrogen fixation have been discovered. Ranges of dinitrogen fixation for 40 legume species in temperate and tropical conditions are summarized and factors that affect fixation, from inorganic N availability to global climate change, are discussed. Several research groups have elucidated the pathways of N transfer between legumes and other plants in mixtures, although the quantities of fixed and transferred N remain difficult to predict. Legume genomes are being determined and this knowledge may help scientists develop plants with improved dinitrogen fixation, although standard plant breeding techniques are beginning to show positive results. Inoculants have been improved by rhizobial strain selection and development, and by better composition and carrier selection, quality control, storage conditions, and methods of application. An emerging area of research and application is the use of legumes for environmental remediation and protection. Deeply rooted perennial legumes actively absorb and assimilate nitrate from subsoil, shallow ground water, and excess fertilizer. Although commercial fertilizer N inputs in agriculture now exceed inputs from dinitrogen fixation, legumes have significant roles to play in improving the sustainability of global food, feed, fiber, and fuel production by virtue of dinitrogen fixation.