Introduction of broadleaf species into monospecific Cunninghamia lanceolata plantations changed the soil Acidobacteria subgroups composition and nitrogen-cycling gene abundances

The introduction of broadleaf species into Cunninghamia lanceolata monoculture plantations could be a sustainable silvicultural practice for restoring soil fertility. We hypothesize that the addition of Phoebe bournei tree species will change soil physicochemical factors and microbial composition. Soil physicochemical factors were determined. Bacterial composition and N-cycling functional genes were investigated by Illumina MiSeq sequencing and qPCR, respectively, and by using a co-occurrence network to explore the connections between the relative abundance of bacteria and soil physicochemical parameters. The introduction of P. bournei into C. lanceolata plantations increased available nutrients in the soil, decreased toxicity (Al3+ level), and slightly increased the soil bacterial diversity. The relative abundance of the total Acidobacteria community, as well as the most abundant subgroups (1, 2, and 3), were significantly higher in the monospecific stands. Different Acidobacteria subgroups behaved differently, i.e., Gp2 correlated negatively with soil nutrients and positively with the Al3+ level, while Gp17 showed the opposite behavior. Quantification of seven nitrogen cycling genes revealed that the nifH, nrfA, and nosZ abundances were higher in the mixed stands, while the nirK abundance was higher in the monospecific plantations. Co-occurrence network analyses showed a strong correlation between soil properties and the bacterial community. P. bournei introduction into C. lanceolata plantations improved soil nutrients and affected the soil bacterial community. The higher nitrogen nifH and nrfA abundance in the mixed stands indicated a higher nitrogen fixation and dissimilatory nitrate reduction potential. Acidobacterial subgroups showed the opposite behavior to P. bournei introduction and could be used as indicators of soil nutrient changes.