Decrease in Nitrogen Fertility of Paddy Soils Induced by Paddy-Upland Rotation and Its Effect on Crop Production.

In Japan, rice plants (Oryza Sativa L.) had been continuously planted in paddy fields. However, because rice production tends to exceed national demand, rice production adjustment by the government has been continued over 30 years. The area of paddy field where other crops are recommended to be cultivated than paddy rice becomes approximately 40% of all the Japanese paddy field. Soybean (Glycine max (L.) Merr.) is a common alternative crop of paddy rice in the northeastern district of Japan. In order to elucidate nitrogen (N) fertility status of soils and crop yields in paddy-upland (irrigated paddy rice and upland soybean) rotation, investigation was conducted in Daisen, Akita, located in the northeastern district in Japan. Top soils collected from 15 sites managed by individual three farmers as well as two experimental fields in the NARCT were examined. Total N and carbon (C) of soils, in which paddy-upland rotation was performed, tended to be lower than those in continuous paddy condition. Available N of soils, which was estimated as the amount of ammonium N extracted by 2 mol L^-1 KCl solution after the incubation under flooded condition at 30^oC for four weeks, was also decreased by paddy-upland rotation. The available soil N was negatively correlated with an upland frequency, which was the rate of number of upland season to total crop season. This decrease in soil N fertility would be caused by difference of N budget between paddy rice and soybean cultivations. Nitrogen budged in paddy rice cultivation would be positive, whereas that of soybean cultivation would be negative. In contrast, yields of soybean and rice did not necessarily reflect the decrease in soil N fertility. In order to sustain the crop productivity, however, the decrease in soil N fertility should be replenished by the application of organic materials.