[Objective] Utilization of green manure crops in winter is an effective practice to maintain high and stable yields in paddy fields in South China, and plays an important role in N management in the rice cropping system. Nitrification is a key process in N cycling and is highly related to N utilization of crops and to N leaching loss as well. Since nitrification process is a potential N loss pathway, it is worthwhile to study impacts of cultivation and incorporation of green manure crops on nitrification process and ammonia oxidizing microorganisms in paddy soil.[Method] A pot experiment having three crops a year, that is, green manure crop, early rice and later rice, was conducted in alkaline paddy soil and designed to have four treatments, i.e. winter fallow-double cropping rice (WF), milk vetch-double cropping rice (MV), ryegrass-double cropping rice (RG) and rape-double cropping rice (RP). Nitrification potential (NP) and recovered nitrification potential (RNP) of the soil in the pots were measured. Relative contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to RNP were explored using specific bacterial inhibitors (Kanamycin and Spectinomycin). Abundances of AOA and AOB were measured using the amoA gene-based real-time quantitative PCR method.[Result] Treatments MV and RG increased the N uptake of early rice and late rice, respectively. Plantation of winter green manure crops, no matter what, significantly lowered NP at the pre-transplanting, tillering and jointing stages of early rice. The NP at the jointing stage of early rice was the highest in Treatment WF, reaching up to 56.15 mg×kg^-1×d^-1. Compared to Treatment WF, the other three treatments were all higher in NH₄⁺-N, and lower in NO₃^--N at most of the sampling stages. Soil NO₃^--N was significantly and positively related to NP. Compared to Treatment WF, all the green manure treatments were higher in copy numbers of AOA and AOB amoA genes at most of the sampling stages, especially the former, which was much higher than the latter. However, AOB was a dominating contributor to RNP at all the sampling stages, contributing relatively 61.02%-82.37%. The abundance of amoA genes had nothing to do with the contributions of AOA and AOB to nitrification. Utilization of green manure crops increased relative contribution of AOB to RNP (RNP_AOB) at the tillering stage of early rice, but decreased that of AOA to RNP (RNP_AOA) at the pre-transplanting and jointing stages of early rice. Soil properties were significantly related to nitrification process, especially soil pH, NH₄⁺-N and NO₃^--N. Application of winter green manure crops decreased soil pH in alkaline soil, which may be one of the main causes of the practice inhibiting nitrification.[Conclusion] Application of winter green manure crops reduces soil NP, which is consistent with the variation of soil NO₃^--N, suggesting that the practice may mitigate the risk of NO₃^--N leaching loss. Therefore, it plays an important role in N management in paddy soil.