抄録
In Japan, Nitrate nitrogen pollution of the ground water by overuse of fertilizer in the upland field had become a serious issue. In order to purify it, the nitrogen removal method using the Topographical chain system was examined. Spring water from upland field was installed to test fields and the experiment was conducted over a long period. The result showed no significant difference for the three kinds of vegetation. In the irrigation period, organic matter supplied by the generation of algae sustained the nitrogen removal. In order to explore the mechanism of the nitrogen removal, experiments were conducted by sampling soil in thin layers. In the rice plot, rice stubbles were put back into the soil did not show significant differences among the layers. In the non-vegetation test plot, the surface and the lower layer had large nitrogen removal. In the shield plot there are no differences in the two upper layers but the lower layers have large value because organic matter below 2 cm layer had not been used for denitrification. Therefore it is thought that denitrification in the paddy fields (wetlands) had happened at around the upper 2cm layer of soil surface
