N 2 -Fixing Cyanobacterial Systems as Biofertilizer

Soil and water surfaces, as well as plant surfaces and tissues are the known locations that harbor free-living phototrophic N2-fixing cyanobacteria. These organisms are known to contribute substantial amounts of fixed nitrogen (20–30 kg N ha−1annually). In continents where rice is the prime crop for majority of the population (amounting to over 40 % of world’s population), these organisms assume great importance. Two third of the total of 180 million tons of fixed nitrogen that gets added to the earth’s surface globally, comes from biological activities mainly contributed by these and other microbes. Rice field ecosystems are ideal for cyanobacterial growth as they provide optimum growth conditions. Azolla-Anabaena symbiotic association, another cyanobacterial system has been exploited as a biofertilizer in many Asian countries. This symbiosis is very important agronomically because its contribution has been estimated to be ~600 kg N ha−1. With the adverse consequences of chemical agriculture, focus on nitrogen enrichment has shifted again to biological nitrogen fixation, especially towards both free-living and symbiotic cyanobacteria. During past few decades, research studies have yielded a large quantity of information on cyanobacterial nitrogen fixation from isolation, molecular understanding and manipulations to large-scale production for agriculture. Substantial research studies have also been devoted towards creating and understanding the artificial associations of cyanobacteria with crop plants. In this chapter, various N2-fixing cyanobacterial systems in light of their use as biofertilizers are reviewed.