The dynamic changes of main nitrogen forms in floodwater of paddy field under different fertilizer rates was studied in Shuangqiao farm,Jiaxing city,Zhejiang province.Soil urease activities was used to explain the mechanisms of nitrogen conversion in floodwater.The results showed that total nitrogen(TN) and ammonia nitrogen(NH~ _4-N) concentration and the ratio of ammonia nitrogen and total nitrogen(NH~ _4-N/TN) in floodwater of each treatment boosted up during the 1~3 days after fertilization and the peak values after the 2nd topdressing was lower than that after the 1st topdressing,and then decreased drastically,maintained at relatively low level after 5 days;it was proved that the first 5 days after fertilization was the most important period of nitrogen especially ammonia nitrogen loss.There was the similar trend of soil urease activities with nitrogen in floodwater.According to the linear correlation analysis,the relationship between NH~ _4-N/TN in floodwater and soil urease activities reached the remarkable and positive relevant level.It was indicated that soil urease activities played a key role in nitrogen conversion in soil-water interface of paddy field.