Abstract: Nitrogen is a very important element and has profound effects on crop fields and environment.Aiming at the problem of high application rate but low use efficiency of nitrogen fertilizer,a field plot experiment was conducted in a field of percogenic paddy soil in the Taihu Lake Region under wheat-rice rotation in the years of 2001 and 2002,in order to estimate NO^-₃-N,NH⁺₄-N,particulate nitrogen（PN） losses with runoff and drainage.Results indicate that the total N loss is low,about 2% of the N applied,and differs very slightly between the wheat season and rice season.Total N lost via runoff and drainage can be sorted as NO^-₃-N and PN,but the proportions of NO^-₃-N,NH⁺₄-N,and PN in the runoff and drainage flows vary from season to season.NH⁺₄-N dominates the loss in the rice season whereas PN in the wheat season.The loss of NO^-₃-N does not differ much between the two seasons.NO^-₃-N,NH⁺₄-N,and PN losses with runoff and drainage vary sharply with the N application rate,cropping season,growing stage,amount of runoff and drainage.

Key words: rice, wheat, runoff and drainage flow, fraction of nitrogen