Changes in soil microbial community are linked to soil carbon fractions after afforestation

The soil microbial community plays an important role in regulating soil organic carbon (SOC) decomposition and maintaining stability in forest ecosystems. However, the interactions between the soil microbial community and soil carbon (C) fractions following afforestation remain poorly understood. In this study, soil samples were collected in an afforested area representing a chronosequence of 42, 27 and 17 years of Robinia pseudoacacia L. succession (RP42yr, RP27yr and RP17yr, respectively), and in farmland (FL) soil for comparison. Illumina sequencing of the16S rRNA and fungal ITS genes was used to analyse soil bacterial and fungal diversity, and the content of C fractions was also measured. Our results indicated that soil C fractions in the afforested RP42yr, RP27yr and RP17yr sites were 34.83–94.11%, 38.52–82.83% and 27.24–89.32% larger, respectively, than in the FL soil. Shannon indices for bacterial and fungal diversity, which ranged from 6.59 to 6.81 and 3.73 to 4.19, respectively, were also larger in the afforested soil. In addition, the dominant bacterial and fungal phyla, including Proteobacteria, Acidobacteria, Chloroflexi, Gemmatimonadetes, Bacteroidetes, Nitrospirae, Verrucomicrobia, Planctomycetes, Armatimonadetes, Cyanobacteria, Chlorobi, Firmicutes, Fibrobacteres, Zygomycota, Basidiomycota and Glomeromycota, were more abundant in afforested soil than in FL soil, whereas Actinobacteria and Ascomycota were more abundant in FL soil than in afforested soil. Spearman's rank correlation coefficients were strong and positive between soil microbial diversity (alpha diversity, Bshannon and Fshannon) and C fractions (P