Abstract: To better understand effects of soil fertility and N fertilization on nitrification in red paddy soil,an incubation experiment was carried out.The objective was to follow changes in nitrification in red paddy soils different in soil fertility and application of N fertilizer.Results show that without fertilizer-N application,nitrification in soils high in fertility was higher than that in those low in fertility.Application of urea significantly increased nitrification in the soils.When it was done at a rate of N 120 and N 360 mg·kg^-1,on average the nitrification rate increased by 75.8% and 357.1% in soils high in fertility and by 52.0% and 146.9% in soils low in fertility,respectively,as compared to CK.In terms of nitrification intensity,the treatments followed the order:soil high in fertility > soil low in fertility,and soils applied with N 360 mg·kg^-1 of urea > soils with N 120 mg·kg^-1 of urea.However,nitrification in red paddy soils was decreased when ammonium sulphate was applied.When its application rate was N 120 and N 360 mg·kg-1,on average,the nitrification rate decreased by 41.8% and 74.7% in soils high in fertility and 3.1% and 65.3% in soils low in fertility,respectively.In terms of nitrification,the treatments applied with ammonium sulphate were in the order:soil low in fertility > soil high in fertility,and soils applied with N 120 mg·kg^-1 of ammonium sulphate > soils with N 360 mg·kg^-1 of the fertilizer.

Key words: Napplication, soil fertility level, red paddy soil, nitrification