Objectives To investigate the response of inter-root microbial communities of winter oilseed rape to different fertilizers, and to determine the appropriate amount of nitrogen fertilizer for oilseed rape, in order to provide a theoretical basis for soil fertilization and regulation of soil ecological functions.

Methods The field experiment was carried out in the oilseed rape planting area (Wuxue), and five treatments were set up under straw-returning conditions, namely, no nitrogen fertilizer (SN0), nitrogen fertilizer N 120 kg/hm² (SN120), nitrogen fertilizer N 180 kg/hm² (SN180), nitrogen fertilizer N 240 kg/hm² (SN240) and nitrogen fertilizer N 300 kg/hm² (SN300). The community structure and functional changes of bacteria and fungi in rhizosphere soil were analyzed using 16S rRNA and ITS high-throughput sequencing methods, while soil physicochemical properties and yield were determined.

Results 1) Compared with SN0, nitrogen fertilizer application reduced available phosphorus (24.63%−35.10%) and available potassium (39.67%−59.77%), and increased total nitrogen (6.18%−14.60%) and alkali-hydrolyzed nitrogen (7.18%−34.99%) in rhizosphere soil. SN180 increased soil total nitrogen and alkali-hydrolyzed nitrogen to the smallest extent, while SN300 increased the most. With the increase of nitrogen fertilizer, organic carbon showed an increasing trend, pH showed a decreasing trend, and yield increased first and then decreased, with SN180 having the highest yield. 2) With the increase of nitrogen application, Chao1 index of bacteria showed an increasing trend. Compared with SN0, SN180 increased the Shannon index of fungi by 7.20%. Application of nitrogen fertilizer changed the community structure of bacteria and fungi. Compared with SN0, SN180 increased the relative abundance of Ascomycetes, Basidiomycetes and Bacteroidetes, decreased the relative abundance of Mortierellomycota and Acidobacteria, and slowed down the decreasing trend of Sphingomonas and Bacillus. Rhodopseudomonas and Filobasidium were enriched. Compared with SN0, nitrogen fertilizer application reduced the abundance of chemical heterotrophic, nitrogen fixation, aromatic compound degradation and chitin decomposition functional bacterial groups, and also reduced the abundance of undefined saprotroph. 3) The analysis of environmental factors and community association network and RDA analysis showed that soil organic C (SOC), alkali-hydrolyzed N (AN), total N (TN) and available P (AP) were the main environmental factors affecting the fungal community, and SOC and AP were the main environmental factors affecting the bacterial community. Achroiostachys was negatively correlated with SOC and positively correlated with AN. Emericellosis was significantly negatively correlated with AN and TN, and significantly positively correlated with pH. SOC was significantly positively correlated with Nocardioides and negatively correlated with Anaeromyxobacteria.

Conclusions From the perspective of yield, community diversity and enrichment of beneficial microorganisms, applying nitrogen fertilizer of N 180 kg/hm² is a reasonable amount for winter rapeseed.