N2‐Fixing Cyanobacteria as Biofertilizers in Rice Fields

Cyanobacteria constitute the largest, most diverse, and most widely distributed group of photosynthetic prokaryotes (Stanier & Cohen-Bazire, 1977). N2-fixing forms contribute by maintaining the fertility of natural and cultivated ecosystems. Currently, research on their agronomical use has almost exclusively focussed on wetland rice. In the 110 000 000 ha of wetland ricefields, N2-fixing cyanobacteria occur as indigenous ubiquitous free-living organisms. Research on ricefield inoculation with cyanobacteria was initiated in Japan by Watanabe et al. (1951). N2-fixing cyanobacteria also occur in symbiosis with the aquatic fern Azolla, which has been used as green manure for rice since the eleventh century in Vietnam and the fourteenth century in China (Lumpkin & Plucknett, 1982). Azolla biotechnology by recombination and sexual hybridization is recent (Wei et al., 1986; Lin et al., 1988).

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