As rice is one of the most important crops in the world, paddy ecosystems has attracted worldwide attention because of their importance in food supply and their eco–functions as wetland. In paddy fields, frequent alternation of wetting and drying required in rice production leads to drastic fluctuation of soil redox potential, thus affecting various microbe-driven aerobic and anaerobic biogeochemical element recycling processes in the soil, including nitrification, respiration and denitrification etc. Since the 1980s, excessive application of chemical fertilizers in various paddy field system management practices in China has brought about a series of environmental consequences, and most of the relevant research efforts in the country have been focused on emission of N₂O, nitrification and denitrification, nevertheless, little is known about the underling microbiological mechanism. Nitrification is an aerobic microbe catalyzed aerobic process, consisting of two steps, i.e. ammonia oxidation and nitrite oxidation. Ammonia oxidation, the rate-limiting step, is mainly catalyzed by ammonia-oxidizing bacteria (AOB) and recently discovered ammonia-oxidizing archaea (AOA). It is generally held that soil moisture ranging between 50% and 70% of water holding capacity (WHC) is the most suitable moisture condition for nitrification, and soils, too dry or too wet, are unfavorable to nitrification. Compared with upland soil, paddy soil is subjected to frequent alternation of drying and wetting, which leads to drastic fluctuation of soil redox potential, thus affecting microbe-driven biogeochemical element recycling processes and metabolic activity and functional succession of soil microbes. Being the major factors affecting soil redox potential, moisture and oxygen therefore become the key factors in the study on nitrogen recycling in paddy soil. To investigate responses of nitrification and nitrifying microbes to change in soil moisture and their underlying microbiological mechanisms, soil sample was collected from a paddy field of alkaline paddy soil, typical of Binhai County, Jiangsu Province, and soil microcosm experiments were set up with 4 moisture gradients including 30%WHC (Treatment 1), 60%WHC(Treatment 2), 90%WHC (Treatment 3), and 2 cm of overlaying water (Treatment 4). Results show that soil Eh was significantly lowered in Treatment 4, and did not differ much between the other three treatments. It varied from 330 to 500 mV, in all the treatments, indicating that on the whole the soils in all the treatments remained in oxidation state that satisfied nitrification basically even under flooded conditions. During the 60-day incubation period, 10 mg kg^-1 NH₄⁺-N was added into soil once every 7 days. Apparent decrease in concentration of NH₄ -N was observed in all the moisture treatments over time, accompanying rapid accumulation of NO₃^--N, which was the most obvious and rapid in Treatment 2 (60%WHC) and then in Treatment 3 (90%WHC). Even though accumulation of NO₃^--N was slow in Treatment 1 and Treatment 4, active nitrification was also observed with the incubation going on. Compared with Treatments 1, 2 and 3, Treatment 4 was much higher in abundance of ammonia oxidizing bacteria (AOB). The dominant AOB bands in DGGE profiles were more distinct and brighter in Treatment 4 than in the others, and the number of AOB bands in the DGGE profiles increased significantly in the late incubation period, that is, from 30 days to 60 days, while AOA (ammonia oxidizing archaea) did not differ much either in abundance or in community structure between the 4 treatments. In addition, AOA dropped sharply in abundance in all the treatments after 15 days of incubation, which may be attributed to the inhibitive effect of continuous nitrogen addition on AOA. All the above described findings indicate that different ammonia oxidizing microbes have different requirements for water and oxygen, and the AOB in this type of soil, being very sensitive to changes in soil moisture condition, are responsible for the nitrification in the tested soil, especially, in waterlogging conditions.