Nitrogen fixation by non-legumes in tropical agriculture with special reference to wetland rice

Abstract

Of the 143 million hectares of cultivated rice land in the world, 75% are planted to wetland rice. Wet or flooded conditions favour biological nitrogen fixation by providing (1) photic-oxic floodwater and surface soil for phototrophic, free-living or symbiotic blue-green algae (BGA), and (2) aphotic-anoxic soil for anaerobic or microaerobic, heterotrophic bacteria. TheAzolla-Anabaena symbiosis can accumulate as much as 200 kg N ha−1 in biomass. In tropical flooded fields, biomass production from a singleAzolla crop is about 15 t fresh weight ha−1 or 35 kg N ha−1. Low tolerance for high temperature, insect damage, phosphorus requirement, and maintenance of inoculum, limit application in the tropics. Basic work on taxonomy, sporulation, and breeding ofAzolla is needed. Although there are many reports of the positive effect of BGA inoculation on rice yield, the mechanisms of yield increase are not known. Efficient ways to increase N2-fixation by field-grown BGA are not well exploited. Studies on the ecology of floodwater communities are needed to understand the principles of manipulating BGA. Bacteria associated with rice roots and the basal portion of the shoot also fix nitrogen. The system is known as a rhizocoenosis. N2-fixation in rhizocoenosis in wetland rice is lower than that ofAzolla or BGA. Ways of manipulating this process are not known. Screening rice varieties that greatly stimulate N2-fixation may be the most efficient way of manipulating the rhizocoenosis. Stimulation of N2-fixation by bacterial inoculation needs to be quantified.