Abstract: To clarify the ecological consequence of efficient forage cropping patterns on soils in the black soil region of the southern foothills of the Greater Khingan Mountains, we used forage oat and forage rape as experimental materials, with a split-plot design being adopted. In the first season, the oat-rape intercropping pattern (FI) was uniformly applied. In the main plots, three double cropping patterns were deployed in the second season: oat monoculture (SO), rape monoculture (SR), and oat-rape intercropping (SI). In the subplots, four nitrogen application levels were set: 0 (N0), 75 (N1), 150 (N2), and 225 (N3) kg·hm^-2. Based on ITS rDNA and 16S rRNA gene sequencing, we investigated the effects of different intercropping and double cropping patterns and nitrogen application levels on soil nutrient contents and microbial community. The results showed that: (1) In terms of community composition, under the N2 treatment, continuous intercropping in the second season (SI) significantly changed the relative abundance of key phyla compared with that in the first season (FI). Specifically, the relative abundances of Gemmatimonadota and Mortierellomycota increased by 9.65% and 10.27%, respectively, while those of Actinobacteriota, Proteobacteria, and Ascomycota decreased by 10.91%, 16.56%, and 3.80%, respectively. (2) In terms of community diversity, cropping pattern and nitrogen application level had a significant interactive effect on the bacterial Chao1 and Shannon indices. Under each nitrogen application level, continuous intercropping in the second season (SI) increased the Chao1 indices of both soil bacteria and fungi compared with that in the first season (FI). The bacterial Chao1 index reached its highest value under the N2 level of the SI pattern. SI and SO patterns had high similarity in bacterial community structure, indicating the dominant regulatory role of oat on soil bacterial community under the intercropping system. LEfSe analysis of differential species further confirmed that the SI pattern had the most significant enrichment effect on characteristic dominant taxa of soil fungal community. (3) Spearman correlation analysis showed that Pseudogymnoascus was significantly positively correlated with soil TN and NO₃^--N, while Humicola and Cladosporium were significantly negatively correlated with soil NO₃--N. These genera can serve as microbial indicators for soil nitrogen cycling. In summary, the continuous intercropping pattern of two seasons per year combined with a nitrogen application level of 150 kg·hm^-2 exhibits significant advantages in optimizing soil microbial community structure, enhancing community diversity, and promoting synergistic nutrient transformation. Our findings provide a solid theoretical basis and technical support for the adjustment of grain-forage structure, efficient forage production, and sustainable utilization of black soil resources in the high-latitude cold pastoral area of the southern foothills of the Greater Khingan Mountains.

Key words: forage oat, forage rape, soil, microbial diversity, community structure