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As the major crops in north China, spring crops are usually planted from April through May every spring and harvested in fall. Wheat is also a very common crop traditionally planted in fall or spring and harvested in summer year by year. This continuous cropping system exhibited the disadvantages of reducing the fertility of soil through decreasing microbial diversity. Thus, management of microbial diversity in the rhizosphere plays a vital role in sustainable crop production. In this study, ten common spring crops in north China were chosen sole-cropped and four were chosen intercropped with peanut in wheat fields after harvest. Denaturing gradient gel electrophoresis (DGGE) and DNA sequencing of one 16S rDNA fragment were used to analyze the bacterial diversity and species identification. DGGE profiles showed the bacterial community diversity in rhizosphere soil samples varied among various crops under different cropping systems, more diverse under intercropping system than under sole-cropping. Some intercropping-specific bands in DGGE profiles suggested that several bacterial species were stimulated by intercropping systems specifically. Furthermore, the identification of these dominant and functional bacteria by DNA sequencing indicated that intercropping systems are more beneficial to improve soil fertility. Compared to intercropping systems, we also observed changes in microbial community of rhizosphere soil under sole-crops. The rhizosphere bacterial community structure in spring crops showed a strong crop species-specific pattern. More importantly, Empedobacter brevis, a typical plant pathogen, was only found in the carrot rhizosphere, suggesting carrot should be sown prudently. In conclusion, our study demonstrated that crop species and cropping systems had significant effects on bacterial community diversity in the rhizosphere soils. We strongly suggest sorghum, glutinous millet and buckwheat could be taken into account as intercropping crops with peanut; while hulled oat, mung bean or foxtail millet could be considered for sowing in wheat fields after harvest in North China.

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Reasonable application of selenium (Se) fertilizer is bene cial for improving Se contents in grains and can affect soil ecology. No study has compared Se fertilizer application methods on bioforti cation, yield, and soil bacterial community. This study investigated the effects of topsoil (T), foliar (S), and soil+foliar (TS) application of Se fertilizer on oats. TS treatment signi cantly increased oat yield compared with the control and S. The Se content in grains was increased in the order of TS > S > T. T and TS increased the nutrients, soil organic matter, activities of urease, alkaline phosphatase, and sucrose, as well as the diversity and abundance of soil bacterial communities. According to PCA analysis, TS and T increased the relative abundance of bacteria involved in the decomposition of organic matter, such as Proteobacteria, Chloro exi, and Bacteroidetes, while reduced Granulicella, Bacillus, Raoultella, Lactococcus, Klebsiella, and Pseudomonas. Furthermore, TS signi cantly increased the relative abundance of Aciditeromonas, Gemmatimonas, Geobacter, and Thiobacter. While, T signi cantly increased the abundance of Lysobacter, Holophaga, Candidatus-Koribacter, Povalibacter, and Pyrinomonas. S did not signi cantly change the bacterial communities. The redundancy analysis revealed that soil nutrients and enzyme activities were positively correlated with the abundance of Actinobacteria, Acidobacteria, Gemmatimonadetes, Bacteroidetes, Planctomycetes, and Chloro exi, but negatively correlated with the abundance of Proteobacteria and Firmicutes. Thus, a combined application of foliar and soil Se proved most conducive for achieving higher yield, grain Se content, and improving bacterial community structure and functional gene expression in rhizosphere soil.

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