Change of storm-runoff load caused by wheat cropping in paddy fields

The storm-runoff loads of nitrogen and phosphorus were observed during seven events in the two experimental small paddy-fields watersheds, the wheat watershed (7.44 ha) and the rice watershed (6.96 ha). The drainage concentrations of total nitrogen (TN) were ranged from 2.70 to 9.86 mg l -1 in the wheat watershed and from 1.56 to 2.18 mg l -1 in the rice watershed. Those of total phosphorus (TP) were ranged from 0.104 to 0.780 mg l -1 and from 0.164 to 0.338 mg l -1 , respectively. In the first storm event after the plowing and basal fertilizer, TN and TP concentration of the wheat watershed was the highest in all events. In this event, the TN and TP concentration of the wheat watershed was 5.3 and 2.5 times higher and storm-runoff load was 4.1 and 1.9 times larger than the same event of the rice watershed, respectively. In this event, nitrate nitrogen (NO 3 -N) and particulate phosphorus (PP) occupied 75% of TN and 47% and TP of the wheat watershed, respectively. As a result, it suggests that the wheat cropping in rice-paddy fields increases the runoff-load of TN and NO 3 -N.