Free-living nitrogen fixers and acetylene reduction in tropical rice field
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In order to assess the contribution of microbial nitrogen fixation to nitrogen fertility of paddy fields in Thailand, the abundance of free-living nitrogen fixers (dilution method) and nitrogen-fixing activities (acetylene reduction method ) were investigated for topsoil (02 cm) and paddy water samples consecutively taken at 40 paddy fields throughout a rice-growing season. Although nitrogen-fixing popu lations and acetylene-reducing activities more or less fluctuated during one cropping season, they were apt to reach the maximum at tillering to heading stages. As in Fig. 1, correlations were obtained between the annual average populations and soil chemical properties. Soil rich in organic matter supported better growth of heterotrophs (Azotobcwter and Clostridiuni butyricum), whereas soils rich in phosphorus generally stimulated the growth of the heterotrophs and blue-green algae. Growth of all nitrogen fixers examined were highly correlated to soil pH within 48. Furthermore, higher populations of C. butyricum, and blue-green algae were counted in soils rich in available nitrogen, suggesting that these two microbial groups ma.inly contribute to enrichment of soil nitrogen. The influences of soil properties on the growth of nitrogen fixers seem responsible for building up of nitrogen-fixing microflora characteristic to each paddy soil group. As