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1.Study on Microflora in a Paddy Rice Field with a Low Dose of Pesticides and Grazing Aigamos (Cross-Breeded Duck)

Kyo Sato(Institute of Genetic Ecology, Tohoku University)

Micrflora were surveyed in the paddy rice soil which was not applied with pesticides and grazed aigamos (cross-breeded duck) in comparison with the soil which was managed conventionally with the application of pesticides (control-plot). The former soil was designated as the aigamo-plot. The growth of rice plant and changes of some of physicochemical properties of the soils were also followed. Main results obtained were as follows:

1) Weeds were controlled by the introduction of the aigamo. Although the growth of riceplant did not differ between the two plots, the yield of rice was a little higher in the control-plot than in the aigamo-plot.

2) pH-values of soil decreased gradually in the control-plot, and it increased in the aigamo-plot. Electric coductivity of the soil elevated greately in the aigamo-plot. Redox potentail changed similarly in both of the plots, suggesting the stirring effect in the soil by the walking of the aigamo and the effect of microbial metabolism of the excreta of the aigamos. Changes in the amounts of NH4-N and available phosphoric acid did not differ between the two plots.

3) The number of total viable bacteria was the largest in the upper soil layer of the aigamo-plot, suggesting a positive effect of aigamos on the growth of the bacteria. The number of Gram-negative bacteria was also larger in the aigamo-plot than in the control-plot. The population of anaerobic bacteria changed similarly in both of the plots. The number of ammonium-oxidizing bacteria increased greatly in the upper soil layer of the aigamo-plot after the introduction of the aigamo, suggesting the effect of the excreta which may form ammonium through microbial decomposition.

4) The bacterial strains were isolated from the both plots before and after the introduction of the aigamos, and grouped based on some of the cytological and physiological characteristics of the strains. The components of the groups were similar in the both plots before the introduction. They, however, differed between both of the plots after the introduction, meaning that the introduction of the aigamo may change the soil microflora.

水田/paddy rice field、土壌のミクロフロラ/soil microflora、合鴨水稲栽培/rice-cultivation by grazing ducks、除草/weed-control、環境/environment