The introduction of Phoebe bournei into Cunninghamia lanceolata monoculture plantations increased microbial network complexity and shifted keystone taxa

Cunninghamia lanceolata is an important timber species and has been widely cultivated in subtropical China. However, monoculture successional patterns lead to multiple ecological problems, such as soil nutrient depletion, biodiversity loss, and forest productivity decline. The introduction of broadleaf species into monospecific C. lanceolata plantations is an effective silvicultural practice driven by the demands to restore soil fertility and produce timber. A comprehensive understanding of how mixed afforestation patterns drive soil microbial community compositions and diversity can aid in the evaluation of the effects of broadleaf trees (Phoebe bournei) on forest soil–plant-microbe ecosystem functions. Therefore, this study aims to explore the effects of conversion from C. lanceolata monoculture plantations to mixed plantations (C. lanceolata-Phoebe bournei) on the soil microbial community by measuring soil physicochemical properties and sequencing the 16S and ITS rRNA genes. The results showed that (1) the effect of forest conversion on bacterial alpha diversity was larger than that on fungal diversity, and the soil microbial community structure of both bacteria and fungi significantly differed between the two interplanting practices; (2) the co-occurrence analysis of bacteria and fungi displayed more connected and complex networks in mixed plantation networks, in which keystone taxa (i.e., Burkholderia and Steroidobacter) might play critical roles in the construction of the microbiome network; and (3) the results from VPA showed that the structure of the soil microbial community was strongly affected by soil environmental factors (e.g., pH). These findings suggest that the introduction of broadleaf species has significant impacts on the soil microbial community, increases the relative abundance of keystone taxa with important functional traits, and drives a more diverse microbial community network in mixed plantations.