Effects of Tree Species Mixing on Soil Microbial Community Structure in Karst Regions of Southwest China

To investigate the effects of tree species mixing on soil microbial community structure in karst regions and its association with environmental variables, this study selected pure Cryptomeria fortunei, pure Liquidambar formosana, and mixed forests in the Zazuo Experimental Forest Farm of Guizhou Province, China, as research objects. We determined nutrient content of leaves, litter, and soil, as well as soil enzymatic activities. Using high-throughput sequencing technology, we analyzed the diversity and composition of the soil microbial community. Mantel tests and variance partitioning analyses (VPA) were used to elucidate the influence of environmental variables on changes in the soil microbial community. The results demonstrated that (1) compared with pure L. formosana forests, tree species mixing significantly increased the Smith-wilson index of soil bacteria, but decreased the Coverage index of soil fungi ( P < 0.05). Significant differences in soil fungal β-diversity were observed among different forest types ( P < 0.05). (2) The dominant bacterial phyla in all three forest types were Chloroflexi, Acidobacteriota, Proteobacteria, and Actinobacteriota, the dominant fungal phyla were Ascomycota, Mortierellomycota, and Basidiomycota. Compared with pure L. formosana and pure C. japonica forests, the relative abundances of Actinobacteria and Ascomycota in mixed forests significantly increased by 37.46% and 37.43%, respectively. In contrast, the relative abundance of Firmicutes in pure L. formosana forests significantly increased by 154.39% compared to that in mixed forests ( P <0.05). (3) Community composition at the phylum level was significantly correlated with leaf and soil nutrient contents, and soil enzymatic activity. The individual explanatory rates of organic carbon, soil nutrients, and soil enzymes on bacterial community variation were 10.08%, 11.14%, and a negative value, respectively, while synergistic effects between enzymes and organic carbon and between enzymes and soil nutrients explained 39.61% and 32.69%, respectively. For fungal communities, the individual explanatory rates of soil nutrients, soil enzymes, and leaf nutrients were 16.53%, 5.72%, and negative, respectively, with synergistic effects between enzymes and leaf nutrients accounting for 0.79%. In conclusion, tree species mixing altered the diversity and composition of soil microbial communities, with environmental variables individually or synergistically influencing microbial community structure. These findings provide a theoretical basis for enhancing ecosystem functions and sustainable management of karst forest ecosystems.