Abstract
Change of chemical and physical soil properties and rice plant's nitrogen-uptake patterns were investigated by field experiment in Hachirogata reclaimed fields. The results obtained are summarized as follows : 1) The oxidized soil layer of rotated paddy fields was thicker than that of continuously rice-cultivated paddy fields. And, the thickness of the oxidized layer decreased by 10 cm each year in paddy fields which were converted from the state of upland fields. At the subsoil in rotated paddy fields, soil structure developed and gaseous phase ratio increased. 2) In rotated paddy fields, root of rice plant extended more deeply and this activity was higher than that in paddy fields. 3) The 2nd year of wetland cultivation after upland use, the amount of absorbed nitrogen in rotated paddy fields by rice plants was more than that in paddy fields. Particularly, the amount of absorbed nitrogen n rotated paddy fields was large at the panicle formation stage and the harvest stage. The recovery rates of basal application were 37.5% in rotated paddy fields and 21.3% in paddy fields. 4) There is a close relationship between the number of spikelets per unit area and the yield of rice. In rotated paddy fields, the increase in rice yield approached a plateau when the number of spikelets exceeded 45,000/m^2. In paddy fields, the increase in rice yield approached a plateau when the number of spikelets exceeded 38,000/m^2. In rotated paddy fields, the percentage decrease of ripened grains was lower than in paddy fields. It is concluded that productivity of rice plants can be enhanced by paddy-upland rotation management in Hachirogata reclaimed fields.
