Introduction: Soil microorganisms are the key factors in elucidating the effects of thinning on tree growth performance, but the effects of vegetation and soil on the species composition and function of soil microorganisms after thinning are still not well elaborated.Methods: The effects of thinning on understory vegetation diversity, soil physicochemical properties and soil microbial community composition were investigated in a thinning trial plantation ofCryptomeria japonicavar.sinensis, including four thinning intensities (control: 0%, LIT: 20%, MIT: 30% and HIT: 40%), and the relationships of the microbial community structure with the understory vegetation diversity and soil properties were assessed.Results: The results showed that thinning had a greater effect on the diversity of the shrub layer than the herb layer. The soil bulk density and the contents of soil organic matter, total potassium and nitrogen increased with increasing thinning intensities. The Shannon and Chao indices of soil bacteria and fungi were significantly lower in the LIT, MIT and HIT treatments than in the control. Thinning can significantly increase the abundance ofProteobacteriaandActinobacteria, and higher thinning intensities led to a higher relative abundance ofAscomycotaand a lower relative abundance ofBasidiomycota,Rozellomycota, andMortierellomycota. Redundancy analysis indicated that soil physicochemical properties rather than understory vegetation diversity were the main drivers of microbial communities, and fungi were more sensitive to soil properties than bacteria. Functional prediction showed that thinning significantly reduced the potential risk of human diseases and plant pathogens, and the nitrogen fixation capacity of bacteria was the highest in the HIT treatment. Thinning significantly increased the relative abundance of cellulolysis and soil saprotrophs in bacteria and fungi.Conclusion: The findings provide important insights into the effects of thinning onC. japonicavar.sinensisplantation ecosystems, which is essential for developing thinning strategies to promote their ecological and economic benefits.

Tree growth, along with soil properties, is greatly affected by forest management. We used a typical sampling to study the impact of four thinning intensities (T1: 0%, 2500 stems ha⁻¹; T2: 20%, 2010 stems ha⁻¹; T3: 30%, 1750 stems ha⁻¹; T4: 40%, 1500 stems ha⁻¹) on the tree growth and soil physicochemical properties and their correlation in Cunninghamia lanceolata plantations. The average annual increments in tree height, diameter at breast height (DBH), and volume increased with thinning intensity, and those of T4 differed significantly (P < 0.05) from those of T1. The average annual stand volume increments of T4 were significantly (P < 0.05) lower than that of T1, while the maximum value presented at T3. However, the effect of thinning in promoting the growth of Chinese fir diminished with time. As the thinning intensity increased, the diameter class distribution of the sample stands moved rightwards. Moreover, thinning improved soil physiochemical properties. The effects of thinning on soil properties in 0–20 cm soil layer were greater than those in 20–40 cm soil layer. There was a positive correlation between available nitrogen, available potassium and tree growth. The results of this study showed that thinning had a potential effect on tree growth and soil properties. The heavy thinning intensity (approximately 1500 stems ha⁻¹) was the optimum for maintaining economic and ecological benefits. However, heavy thinning significantly reduced stand volume. From the perspective of improving stand volume and biomass, a moderate thinning intensity (approximately 1750 stems ha⁻¹) could be considered for adoption.