【Objective】 Biochar has increasingly been used as soil amendment to increase soil carbon storage and improve soil chemical and biological properties. However, so far little has been found in the literature talking comparatively about effects of returning maize stalk, directly or as biochar on soil properties, nitrification and ammonia oxidizer communities. 【Method】 A 3-year field experiment was conducted in a field of typical fluvo-aquic soil under the wheat/maize rotation–system commons in the North China Plain. The field experiment was designed to have four treatments: Treatment S (application of straw); Treatment B (application of straw-derived biochar); Treatment SB (half straw and half straw-derived biochar) and Treatment CK (no straw or biochar applied). Soil samples were collected in the wheat and maize seasons for analysis of soil properties and community structure and diversity of, ammonia oxidizing archaea and bacteria with the terminal restriction fragment length polymorphism (T-RFLP) and clone library techniques.【Result】It was found that during the wheat season Treatment B significantly enhanced soil pH, soil organic carbon (SOC) and readily available potassium (AK), while decreasing soil bulk density, but did not affect the contents of total nitrogen (TN), NO₃^--N or soil, water as compared to Treatment S, and PNR in Treatments B and SB was measured to be 0.58 and 0.49 μg·h^-1·g^-1, respectively, significantly higher than that in CK (0.22 μg·h^-1·g^-1), but did not differ much from that in Treatment S (0.40 μg·h^-1·g^-1). During the maize season, Treatment B significantly increased soil water content, SOC and AK, and PNR on the whole was relatively lower in the maize season than in the wheat season, regardless of treatment, and much higher in Treatment B (0.27 μg·h^-1·g^-1) than in Treatments CK and S (P<0.05). During the wheat season, Potential nitrification rate (PNR) was closely related to AK, NH₄⁺ and soil bulk density (P<0.05), but not so to ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) community structure. During the maize season, PNR was significantly related only to SOC and to AOB community structure as well. Redundancy variance analysis (RDA) shows that soil SOC, bulk density, pH and AK are the main factors significantly affecting community structure of soil AOA and AOB, which explained 76.4% and 75.5% of their total variation. Phylogenetic tree analysis shows that all archaeal amoA gene sequences were exclusively clustered with group 1.1b (Nitrososphaera); and almost all the bacterial amoA sequences in the fluvo-aquic soil belonged to Nitrosospira cluster 3.【Conclusion】To sum up, application of biochar stimulates nitrification in the sandy fluvo-aquic soil by changing soil SOC, bulk density, pH and AK. The finding in the sequencing and RDA further indicate that biochar application triggers changes of AOA and AOB in community structure. The finding may serve as theoretical references in rational utilization of straw and its biochar.