Abstract
When paddy soil receives air-dry pretreatment, the amount of nitrogen mineralization after submergence increases drastically. This phenomenon is called "air-drying effect on ammonification" (air-drying effect). In this paper, we studied the relationship between the degree of dehydration before submergence and the soil nitrogen mineralized after submergence in order to develop a method for forecasting soil nitrogen mineralization pattern during the rice growing period. The results obtained are summarized as follows: 1) When the soil was air-dried below certain water potential before submergence, "air-drying effect" occurred and the amount of mineralized nitrogen increased linearly with the decrease of soil water content. 2) We call this critical water potential "K point." Water potential of "K point" was about pF 4 for every paddy soil studied, although the water content of "K point" was 26-33% for fine-textured soil and 16-18% for medium- to coarse-textured soil. 3) The amount of nitrogen mineralized can be regressed with a straight line by soil water content below K point. Slope of the regression line was specific to each soil studied. Slope was affected neither by nitrogen fertilization nor by the straw incorporation. 4) Promotion of mineralization by "air-drying effect" completed until the cumulative temperature above 15℃ reached 150-210 degree・days. This cumulative temperature corresponds to the vegetative stage of rice plants. Thus, it is important to estimate the amount of "air-drying effect" before submergence to determine the amount of basally applied nitrogen fertilizer. 5) Aerobic preincubation at 25℃ for 4 weeks reduced the "air-drying effect" after submergence considerably. However, when the preincubation temperature was set at 10℃, the loss of "air-drying effect" was only 20%, indicating the persistence of "air-drying effect" in the field paddy soil where soil temperature is low before submergence.
