Ammonia oxidization is a critical step in the soil N cycle and can be affected by the fertilization regimes. Chinese milkvetch (Astragalus sinicus L., MV) is a major green manure of rice (Oryza sativa L.) fields in southern China, which is recommended as an important agronomic practice to improve soil fertility. Soil chemical properties, abundance and community structures of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in a MV-rice rotation field under different fertilization regimes were investigated. The field experiment included six treatments: control, without MV and chemical fertilizer (CK); 100% chemical fertilizer (NPK); 18 000 kg MV ha -1 plus 100% chemical fertilizer (NPKM1); 18 000 kg MV ha -1 plus 40% chemical fertilizer (NPKM2); 18 000 kg MV ha -1 alone (MV); and 18 000 kg MV ha -1 plus 40% chemical fertilizer plus straw (NPKMS). Results showed that NPKMS treatment could improve the soil fertility greatly although the application of 60% chemical fertilizer. The abundance of AOB only in the MV treatment had significant difference with the control; AOA were more abundant than AOB in all corresponding treatments. The NPKMS treatment had the highest AOA abundance (1.19 × 10 8 amoA gene copies g -1 ) and the lowest abundance was recorded in the CK treatment (3.21 × 10 7 amoA gene copies g -1 ). The abundance of AOA was significantly positively related to total N, available N, NH4 + -N, and NO3 --N. The community structure of AOA exhibited little variation among different fertilization regimes, whereas the community structure of AOB was highly responsive. Phylogenetic analysis showed that all AOB sequences were affiliated with Nitrosospira or Nitrosomonas and all AOA denaturing gradient gel electrophoresis (DGGE) bands belonged to the soil and sediment lineage. These findings could be fundamental to improve our understanding of AOB and AOA in the N cycle in the paddy soil.Key words: Abundance, ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), Astragalus sinicus, community structure, Chinese milk vetch, soil chemical properties. INTRODUCTIONAppropriate fertilizer application is an important management practice to improve soil fertility. In recent years, due to the rapid population growth and continuous decline in the amount of cultivated land area, the application rate of fertilizer keeps on rising in tropical and subtropical regions of China in order to obtain high crop production. However, the long-term inappropriate fertilization has caused many environmental problems, such as high acidity, low nutrients in cultivated land and unbalanced ecosystem (Chen et al., 2010). Recently, soil quality has gained much attention as a result of environmental issues related to soil degradation and production sustainability under different farming systems (Galantini and Rosell, 2006). Chinese milk vetch (Astragalus sinicus L., MV) is considered as the most popular green manure in paddy field of South China, due to its high N-fixing potential and better growth under wet paddy soi...