On the basis of seven long-term located experiments in paddy fields

analysis was made on the change of heavy metal content and its availability in soil of 0~20 cm which were treated with four different fertilization trails detailed as fertilizing by no fertilizer

NPK

middle-level manure(30% organic N 70% chemical N) and high-level manure(60% organic N 40% chemical N).The results indicated that there was a bigger risk for paddy soils to be polluted by heavy metals under the treatment with long-term combination of chemical fertilizers and farmyard manure.Little change happened on the contents of total and available Zn

Cu

Cd

Pb and their activity under NPK treatment.But total and available Zn

Cu

Cd contents and their activity significantly ascended under MOM or HOM treatment.The most increase was under HOM treatment

at a rate of 6.1%

18.7%

8.3% respectively for total Zn

Cu

Cd contents

at a rate of 87.3%

65.8%

41.4% respectively for available Zn

Cu

Cd contents

and at a rate of 77.5%

32.0%

29.8% respectively for activity of Zn

Cu

Cd

compared with CK treatment.This increase was mainly caused by "activation" effect of farmyard manure.In addition

soil total Cd content overtopped Soil Environmental Quality Standard II(0.30 mg/kg) in all experiment sites

indicating that there was possibly prevalent Cd pollution in these paddy soils.