Abstract
Rice grain production in a long-term unfertilized paddy field was compared with that in an adjacent paddy field which had been supplied with the standard level of fertilizers in 1980-1998 to elucidate the mechanism of maintained grain production in the unfertilized field. Average grain yield (brown rice) in the unfertilized paddy field was 382.7 g m-2 that in the fertilized field in the adjacent field was 480.0 g m-2 indicating that 80% of grain production was constantly maintained without supplying any nitrogen fertilizer. The amount of nitrogen absorbed by rice plants for producing 1 g grain was estimated to be 14.1 mg, 55% higher than that in the fertilized field in terms of grain production efficiency. The amount of nitrogen absorbed by rice plants per year in the unfertilized field was calculated to be 5.4 g m-2 This amount of nitrogen should have been supplied annually to maintain the stable grain production for the period of 18 years. Quantitative analysis of nitrogen in the unfertilized field demonstrated that 1.4 g m-2 nitrogen was supplied from irrigation water containing suspended solids, 0.68 g m-2 biological fixation, and 9.0 g m-2 from soil, respectively, to maintain the stable grain production. Total nitrogen of soil in the unfertilized paddy field had been maintained at a constant level during these 18 years, suggesting that grain production of around 380 g m-2 (brown rice) could be supported without fertilization for an extended period of time.
