The spatial distribution and drivers of soil microbial richness and diversity in a karst broadleaf forest

Soil microorganisms regulate ecosystem function and aboveground community dynamics in terrestrial ecosystems. However, our current understanding of the drivers of soil microbial diversity lags our understanding of macroorganisms. Here, we used Illumina sequencing of 16S and ITS rRNA genes to explore which factor(s) controlled soil microbial (i.e., bacteria and fungi) richness and diversity in a 25-ha karst broadleaf forest in Southwest China. Across the plot, all bacterial and fungal richness (number of OTUs) and diversity indices (Shannon diversity and phylogenetic diversity) showed strong spatial autocorrelations. Soil microbial richness and diversity indices displayed a unimodal pattern from south to north in the karst forest, with peaks in the low and middle areas of the plot. Slope was found to be the best predictor of soil bacterial and fungal richness and diversity indices. Soil pH was negatively related with bacterial richness, and with bacterial and fungal phylogenetic diversity. Tree Shannon diversity and density together explained much of the variations in fungal OTUs and diversity indices. Spatial factors explained much less of the variation in soil microbial richness and diversity indices than the selected environmental variables, which indicated that habitat heterogeneity rather than dispersal limitation played an important role in soil microbial richness and diversity in the karst forest. In conclusion, slope was the major driver of the spatial distribution of soil microbial richness and diversity in the karst forest due to its effects on plant (i.e., tree Shannon diversity and tree density) and soil characteristics (i.e., soil pH and available phosphorus).