Effects of tree species and soil properties on the composition and diversity of the soil bacterial community following afforestation

•Soil bacterial community structure and composition following afforestation mainly affected by tree species composition and tree traits.•Changes in bacterial community composition were driven by the Actinobacteria, Bacteroidetes and Nitrospirae phyla.•The effect of afforestation on the soil bacterial diversity was larger than its effect on bacterial richness. Afforestation with single and mixed tree species on cropland alters soil properties and the composition and diversity of soil bacterial community. We investigated changes in soil properties and the soil bacterial community composition in the topsoil (0–20 cm) following afforestation on cropland on the Loess Plateau. Forests of six types ranging from 23 to 25 years in age were chosen in this study: Robinia pseudoacacia (RP), Platycladus orientalis (PO), Pinus tabuliformis (PT), Robinia pseudoacacia + Platycladus orientalis (RPPO), Robinia pseudoacacia + Pinus tabuliformis (RPPT), and Platycladus orientalis + Pinus tabuliformis (POPT). Afforestation with different tree species altered the soil nutrient contents, with total nitrogen (TN) and ammonium nitrogen (NH4+) contents increased by 18.28–87.10% and 13.97–141.34%, respectively, and available phosphorus (AP) content decreased by 56.09–82.13% compared to cropland sites. Cropland afforestation reduced the soil bacterial richness and core bacterial genera abundance (relative abundance ≥ 0.1%), and significantly altered the bacterial diversity. Soil properties explained 73.41% of the total variation in the core soil bacterial community composition at the genus level, and soil organic carbon (SOC), TN, nitrate nitrogen (NO3−), NH4+, AP, pH and the carbon:phosphorus (C:P) ratio were significantly correlated with first two RDA (redundancy analysis) axes. The results of hierarchical clustering indicated that the soil bacterial community composition in the RPPO, RPPT and POPT sites was largely affected by Platycladus, Pinus tabuliformis and Platycladus, respectively. Our observations suggest that soil bacterial community composition and diversity following afforestation were mainly affected by tree species, followed by soil parameters. The effect of afforestation on the soil bacterial diversity was larger than its effect on bacterial richness.