Long-term effects of vegetation and soil on the microbial communities following afforestation of farmland with Robinia pseudoacacia plantations

Soil microorganisms play an important role in the functional connection of nutrient cycling between plants and soil ecosystems, but the exact responses of microbial communities to change in plant characteristics and soil properties remain uncertain. We analyzed the soil microbial communities of afforested tree stands of varying ages to assess the effects of vegetation and soil quality on soil microbial communities. We studied Robinia pseudoacacia plantations in the Loess Plateau for three age classes (RP13, RP29, and RP44), and compared results from these classes to one farmland (FL) control. The responses of soil microbial communities to changes in understory flora and soil physicochemical properties, including the Shannon index, richness, coverage, and biomass were analyzed. Compared with FL, afforestation significantly increased the Shannon index, species richness, coverage, and biomass of understory plants, which were highest in RP29. The concentrations of soil organic carbon (SOC), total nitrogen (TN), nitrate nitrogen (NO3--N), and ammoniacal nitrogen (NH4+-N) were highest in RP44. The soil microbial community diversity index was significantly affected by afforestation. The relative abundances of bacterial populations were also affected, with Proteobacteria being dominant in RP29 and Actinobacteria being dominant in FL. Similarly, in fungi, the relative abundance of Basidiomycota was highest in FL, while the relative abundances of Zygomycota were highest in RP29. Pearson correlation analysis revealed that the Shannon and Chao indices of soil fungi were significantly positively correlated with SOC and NH4+-N. RDA showed that SOC, NH4+-N, and soil water content (SWC) were significantly associated with soil bacterial community composition. Soil fungal community composition were significantly affected by the Shannon and Richness indexes of understory flora. The results showed that characteristics of understory flora, soil properties, and soil microbial community composition were all affected by afforestation. The composition of the fungal community was correlated with changes in the diversity of understory flora. The changes in nutrients such as organic carbon and inorganic nitrogen was the key to the changes of soil microorganisms as plant colonization.