Microflora related to the nitrogen cycle in the tropical paddy soils

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Abstract
Paddy soils collected in 103 sites of Thailand consisted of surface soil (0-1 cm) and subsurface soil (1-10 cm). Collections were made twice, in the middle of the rainy season and in the middle of the dry season. The counts of 12 groups of microbes related to the nitrogen cycle were determined. In the soils sampled in the rainy season the population level of aerobic bacteria per 1 g of dry soil ranged from 107 to 106. For other microorganisms, the values were as follows: actinomycetes—l04 to 104, anaerobic bacteria—l05, cellulose decomposer—104, denitrifier—105, ammonifier—104 to 104, nitrite oxidizer—103, ammonia oxidizer-105, Clostridium-105 to 104, nitrogen-fixing blue green algae, Azotobacter and purple nonsulfur bacteria-103, respectively. Populations of all 12 groups examined were larger in the oxidized surface soil than in the subsurface soil and, in particular, the populations of aerobic nitrite oxidizer and photosynthetic purple nonsulfur bacteria were more than 4 times higher than those in the subsurface soil. The populations of microorganisms, except for cellulose decomposer, decreased in the dry season compared with those in the rainy season. The largest decrease was recorded in the population of nitrite oxidizer which dropped to the level of 1/26 especially in the surface soil, but only to 1/5 in the subsurface soil, and was followed by the population of purple nonsulfur bacteria. The populations of 12 groups of microorganisms except those of nitrite oxidizer and purple nonsulfur bacteria recorded the largest value in fresh water alluvial soils with rather high organic matter content and heavy clay followed by low hurnic gley soils with low organic matter content, while the smallest value was generally recorded in brackish water alluvial soils with low available phosphorus content and low pH value. The populations of nitrite oxidizer were the largest in low hurnic gley soils where the oxidized layer developed, while those of purple nonsulfur bacteria showed the largest value in brackish water alluvial soils. The population of nitrite oxidizer was noticeably higher in tropical than in Japanese soils. The ratio of aerobic bacteria to actinomycetes in the tropical soils showed a small value, namely 1.60 in the rainy season, while that of Japanese soils was about 10.