Objectives Wheat straw directly returning to field before summer maize sowing is common practice in winter wheat–summer maize rotation system in North China Plain. Suitable nitrogen managements on maize in the system were studied from enzymatic activity and bacterial communities, aiming to deeper understanding on the mechanism in increasing yield and fertilizer efficiency.

Methods A field experiment was conducted consecutively for five years in Henan Academy of Agricultural Sciences. The treatments included zero fertilization (CK), mineral N application (the ratio of basal∶topdressing nitrogen was 1∶1, N), and whole wheat straw returned into field with mineral N separately applied in ratio of basal to topdressing of 1∶1 (SN1), 1∶1.5 (SN2) and 1∶2 (SN3). Soil samples were collected from topsoil (0–20 cm) at silking stage and harvesting stage of maize in 2016. The soil basic physicochemical properties were determined by conventional method, enzyme activities by fluorescence microplate enzyme assays, and soil bacterial community structures by PCR and Illumina Miseq high-throughput sequencing platform.

Results The three straw incorporation with N management treatments improved maize yield and nitrogen use efficiency. Compared with N treatment, SN1 treatment increased the yield by 9.98%. The nitrogen use efficiency of SN1 and SN2 treatments was 9.83 and 5.10 percentage points higher than that of N treatment, respectively. Compared with N treatment, the highest increase in soil total-N, NO₃^–-N, NH₄⁺-N, available P and available K contents at silking stage and harvesting stages were all in SN1; all the enzyme activities, except phosphatase at silking stage, were significantly enhanced in SN1 treatment; the diversity of bacterial community was significantly increased in SN1 at the silking stage. The dominant populations in the phylum and class levels were Proteobacteria, Actinomycetes and α-Proteobacteria, Actinomycetes, respectively. LEfSe analysis showed that the maximum LDA（linear discriminant analysis) value of individual SN treatments was α-Proteobacteria and γ-Proteobacteria at silking stage and harvesting stage, respectively; while that was Pcidobacteria at silking stage and Blastomonas at harvesting stage. Canonical correspondence analysis (CCA) showed that soil pH (P = 0.002), organic matter content (P = 0.004) at silking stage and soil pH (P = 0.03), nitrate N (P = 0.036), available K (P = 0.044) at the harvesting stage significantly affected bacterial community structure.

Conclusions Soil pH, organic carbon, nitrate nitrogen and available potassium contents are the main factors influencing the bacterial community structure. Under straw incorporation, the ratio of basal application to topdressing of nitrogen fertilizer significantly effects the soil enzymatic activities, the diversities and richness of bacterial community and the bacterial community structure. When the ratio of basal ∶topdressing N is 1∶1, the activities of most soil nutrients and enzymes, the relative abundance of α-Proteobacteria, Chloroflexi and TK10 (unclassified）will be increased significantly, which implies the fast decomposition of returned straws and the effective nutrition regulation by the formed organic matters, thereby the increase of maize yield and nitrogen use efficiency.