Continuous cropping of soybean alters the bulk and rhizospheric soil fungal communities in a Mollisol of Northeast China

Continuous cropping of soybean causes soil degradation and decreased soybean yield in Northeast PR China. However, the mechanism underlying the effect on soil fungal communities and the relationship between these effects and cropping systems have not yet been fully elucidated. In this study, we comparatively investigated the abundance, diversity, and structures of fungal communities in the bulk and rhizospheric soils of soybean grown in two cropping systems, that is, continuous cropping (CC) of soybean and cropping rotation (CR) with maize using real-time Polymerase Chain Reaction (PCR) and Illumina MiSeq sequencing methods. The results showed that the rhizospheric soil fungal abundance was significantly higher in CC than in CR, but the opposite trend was observed in bulk soils. CC decreased the fungal alpha diversity in both bulk and rhizospheric soils compared with CR. The fungal community compositions were distinctly different between CC and CR in both bulk and rhizospheric soils, and the composition and diversity in bulk soils were predominantly affected by the soil carbon to nitrigen (C/N) ratio. The soil fungal functional groups varied between the two cropping systems; CC increased the relative abundances of the potentially pathogenic fungi Fusarium oxysporum and Lectera longa and beneficial fungi Mortierella sp. and Paecilomyces lilacinus. In contrast, CR increased the relative abundance of Ustilago maydis, which is a pathogen that causes corn smut. Our findings showed that CC increased the abundances of plant pathogens, which is harmful to soybean growth; in contrast, the abundances of some beneficial fungi also increased, which suggested that suppressive soils might be developed after long-term CC.