Influences of puddling, fertilizer and kinds of soils on the nitrogen and phosphorus concentration in field water

The concentration of nitrogen and phosphorous in field water standing on soil in twenty-four pots was measured 1, 3, 5 and 7 d after the experiment was conducted. The experimental conditions were as follows : 1. Puddling treatment (Unpuddled or puddled), 2. Fertilizer application (No fertilizer, phosphate fertilizer or synthesized chemical fertilizer) and 3. Kinds of soils (Gley, Kuroboku, Yellow or Masa). Twenty cm of each type of air-dried soil was put into a pot (500 c square m). Fertilizer was incorporated to the plow layer. Water was poured gently until the water level reached 10 cm above the soil. Water and the upper soil (10 cm) were puddled together for 5 min by using a mixing machine. Pots were set in a room controlled at 20 deg C. The experiment was carried out three times. The analysis of variance shows that the effective factor for nitrogen and phosphorous concentration is the interaction of puddling operation with fertilizer application. It seems that for the T-N and D-N concentration, the influence of fertilizer application is more significant than that of puddling operation, while the influence of puddling operation is more significant for the P-N, T-P, D-P and P-P concentration. The mean value of concentration in the puddled and fertilized pots measured after 7 d was 9.5 mg L sup-1 for T-N and 0.74 mg L sup-1 for T-P. These values were higher than the irrigation water quality standard, so it seems that it is necessary to continue to reduce runoff from paddy field, 7 d after puddling operation may not be enough time for the water to become purified.