An expermient using monolith lysmieter under a paddy field irrigation-drainage system was conducted to monitor and analyze soilnutrient balance in South Jiangsu Plain.Results showed that the paddy field ecosystem consumed 890~1 320 mm irrigation water in each season,and it might purify nutrients by absorbing 10.7~12.3,6.8~9.2 and -1.2~2.0 kg hm^-2 of NH₄⁺-N,NO₃^--N and TP (total phosphorus),respectively in all fertilization treamtent,regard less of application rate.Although nutrient leaching rate in creased with the fertilization rate,the net nutrient purifying rate did not seem to be closely related to it.Application of straw had a little effect on purifying rate,while application of pigmanure weakened the purifying function sign ificantly,and could even be deemed as a "source" of phosphorus in surrounding water bodies During the rice growing period,paddy field played the role of purifying the water of NH₄⁺-N and NO₃^--N and of TP as well,except for in the treamtent of adding pig manure NO₃^--N,however,was liable to leach in the early rice growing period.