The Biological Nitrogen Fixation Systems Adopted in Rice Paddy Fields in China

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The rice paddy fields in China are mainly distributed in the southern provinces in the subtropical and temperate climatic regions. The Yangtze River Valley is the center of rice plantation which stretches from the east coast to the western mountainous areas of Szechuan and Yunnan provinces. This vast area of more than 20 million hectares of rice paddy fields have gone through several hundreds of years’ of agricultural practices of rice-green manure and rice-wheat or rape types of paddy/dryland rotational farming. A type of humus-rich and high N-content rice paddy soil has been developed with high productivity for grains (Xiong and Li 1987). Farmers used to grow a crop of leguminous green manure, milk vetch (Astragalus) or vetch (Vicia) after rice harvesting in late autumn for the effect of symbiotic nitrogen fixation of Rhizobium and to use it as base manure to supply organic matter and nitrogen for the next rice crop (Chiao 1986). During the growing period of rice plants, in addition to the different naturally developed filamentous and unicellular nitrogen-fixing blue-green algae in the stagnant water layer of the rice field, farmers also grow Azolla in the rice fields serving as top-dressing green manure to supply nitrogen and other nutrients (Liu 1981). Azolla infected with a blue-green alga, Anabaena azollae, is a very effective symbiotic nitrogen fixation system on shallow water surface in temperature regions. Thus the opportunities of free-living and symbiotic blue-green algae for nitrogen fixation come into full play in Chinese rice paddy fields.