Abstract
The role of subsoil on nitrogen uptake by rice plants was investigated by field experiments in Hachirogata reclaimed paddy fields. Also, the fitness of models for predicting nitrogen mineralization of plow layer and subsoil to the nitrogen uptake by rice plants was investigated. The results obtained were as follows : 1) Amount of nutrient uptake by rice plants form subsoil was large in rotated paddy fields. 2) At the subsoil in rotated paddy fields, progress of soil reduction was slower and amount of mineralized nitrogen was more than in continuously rice cultivated paddy fields. 3) Amount of nitrogen uptake by rice plants form soil at harvest time was 8.5 g/m^2 in continuously rice cultivated paddy fields and 15.8 g/m^2 in rotated paddy fields. In continuously rice cultivated paddy fields, 20% of nitrogen uptake was absorbed from subsoil and 65% in rotated paddy fields. 4) The process of nitrogen mineralization of plow layer and subsoil and 65% in rotated paddy fields. 4) The process of nitrogen mineralization of plow layer and subsoil was fitted to a single exponential model in continuously rice cultivated paddy fields and rotated paddy fields. 5) After young panicle formation stage, the amount of nitrogen uptake by rice plants in rotated paddy fields was more than the estimation of soil nitrogen mineralization of plow layer and was fitted to the sum of nitrogen mineralization from plow layer and subsoil. It is concluded that the amount of nitrogen uptake by rice plants in rotated paddy field can be enhanced by the nitrogen uptake from subsoil through the penetration of roots into the subsoil.
