Abstract
The field studies were carried out at three rice paddy fields in Okayama Prefecture, in the western part of Japan, since 1992 to make clear the effect of a continuation of no-tilled direct seeding cultivation on the oxidation and reduction potential in the topsoil of paddy fields (soil Eh), comparing that from conventional tilled transplanting rice paddy fields. Soil Eh, the percolation rate of ponding water and the thickness of organic mater layer on the topsoil were measured routinely at the rice paddy field in Akasaka (Haplic Gray Lowland soils), Suimon (Motted Gley Lowland soils) and Sanyoh (Gleyed Gray Lowland soils), where the application of rice straw in the no-tilled direct seeding fields has been continued since 1993,1993 and 1992,respectively. The major findings were as follows : 1) The organic matter was accumulated on the surface of the topsoil by continuing the no-tilled direct seeding rice cultivation, where rice straw was scattered on the soil surface every year, and its thickness reached about 16 mm after 8 y in Sanyoh. 2) It is generally accepted that the percolation rate of ponding water becomes larger after continuing the rice cultivation with no-tilled direct seeding, but the field measurement revealed that it did not always become larger in the rice fields of Suimon and Sanyoh where the level of irrigation water was high during irrigation. 3) The soil Eh decreased with the continuation of no-tilled direct seeding cultivation, and there was little difference in the soil Eh between the no-tilled direct seeding rice cultivation. Since then, the value of soil Eh after submergence quickly decreased from the surface to a depth of 5.5 cm in the topsoil of no-tilled direct seeding cultivation, and was almost the same as that of the tilled transplanting field. 4) The Eh of the surface soil became quite low and reductive in the rice paddy field where the organic matter was accumulated on the surface of the topsoil by continuing the no-tilled direct seeding cultivation in Akasaka, although the percolation rate was great (52.5 mm d ^<-1>).
