Abstract:The activities of invertase,urease,acid-phosphatase and dehydrogenase,the number of bacteria,actinomycetes and fungi and the contents of microbial biomass C and N (MBC and MBN) in soils sampled at the booting,heading and milky stages of rice were investigated using pot experiment under three irrigation methods including controlled irrigation (CIR),intermittent irrigation (IIR),flooding irrigation (FIR) under three ratios of organic to inorganic nitrogen (N) such as 60% inorganic N+40% organic N (F1),80% inorganic N+20% organic N (F2),100% inorganic N (F3).Results showed that the enzymatic activity,the number of microorganism and the contents of MBC and MBN in soils were increased from booting to heading stages with a peak of increase at the heading stage and decreased from the heading to milky stages.Comparing with FIR treatment,CIR and IIR treatments generally increased the enzymatic activity,the number of microorganism and the contents of MBC and MBN in soils.Combined application of organic and inorganic N generally had higher enzymatic activity,the number of microorganism and contents of MBC and MBN in soils than those of only inorganic N.F1 treatment had high organic fertilizer proportion.Thus water-saving irrigation method (CIR and IIR) could effectively enhance the microorganism activity of paddy soil under combined application of organic and inorganic N,F1 treatment.