Abstract
Phosphate accumulated in paddy soils is released from the soils to flooded water when pH of the flooded water is much increased by photosynthesis of periphyton on the soil surface. The growth of periphyton depends on nitrogen in the soils. We investigated the effect of nitrogen form in a paddy soil on change in pH of flooded water caused by photosynthesis of periphyton at 15℃. In this experiment, we applied 0g/m2, 5g/m2 and 10g/m2 of nitrogen as potassium nitrate and the same amount of nitrogen as urea to each soil. As the result, the pH of the flooded water on the soil to which nitrogen was not applied increased from 5.4 to 7.1. The increase in the pH of the flooded water on those soils in which 5g/m2 and 10g/m2 of nitrogen as urea were applied was almost identical to the increase in pH of the flooded water on the soil to which nitrogen was not applied. However, the pH of the flooded water on those soils in which 5g/m2 and 10g/m2 of nitrogen as potassium nitrate were applied increased from 5.1 to 8.1 and 9.7, respectively. This suggests that when periphyton uses nitrate nitrogen in the soil, photosynthesis is more activated, increasing pH of the flooded water.
