Effects of winter flooding on the soil nitrogen fractions and organic rice growth in a paddy field with low nitrogen fertility

We investigated why winter-flooded, organically cultivated paddy fields have high productivity by conducting field experiments in which winter-flooded (WF) and winter-drained (WD) main plots that each contained no fertilization and standard fertilization subplots were established from 2013 to 2015. We found that the WF treatment significantly increased rice yield and yield components, such as the number of ears, grain number per panicle, and grain number per square meter. Furthermore, the plant height, stem number, and leaf color of the rice were higher with the WF treat-ment than the WD treatment through most of the growth period. In early spring, a surface layer with a light clay texture formed above the plow layer, which had a sandy loam to sandy clay loam texture. This surface layer was approximately twice as thick in the WF field (1.8-4.2 cm) than in the WD field (0.5-2.2 cm) and contained three to five times higher contents of inorganic + available nitrogen (128-171 mg/kg), sulfuric acid-extractable nitrogen (68-72mg/kg), 1 M NaOH-extractable nitrogen (1729-2078 mg/kg), and total nitrogen (2859-3301 mg/kg) than the plow layer, which could be attributed to the movement of the clay fraction from the plow layer to the surface layer. The contents of these nitrogen fractions were not affected by the WF treatment, but the total amount of inorganic + available nitrogen in the entire soil layer was higher in the WF field than in the WD field. Thus, the increased thickness of the surface layer of the WF field combined with the increased available nitrogen content in the surface layer may explain why WF treatment increases the rice yield in paddy fields in southwestern Japan with low nitrogen fertility.