Mature tiankengs are the soil microbial hotspots of the karst regions

Human activity accelerates the soil degradation and biodiversity loss of karst regions. Tiankeng, the negative geological structure surrounded by walls, as refugia conserves the endemic species of karst regions. The mature tiankengs have integral walls but the walls are broken in degraded tiankengs. Although the biodiversity in degraded tiankengs was investigated, studies exploring the role of mature tiankengs in the karst region are lacking entirely, however, it is key to biological conservation and the maintenance of ecosystem stability in the karst region. Soil samples were taken from four mature tiankengs in southwest China and the biodiversity of bacteria and fungi were determined using high-throughput sequencing. Then the functional guilds of microorganisms were assigned according to the databases of FAPROTAX and Funguild. Generally, the soil nutrient contents, species diversity, and endemic species richness of bacteria and fungi were higher in the tiankeng than in outside, i.e. karst land. Compared with karst land, the species abundance of 69 and 98 bacterial and fungal genera, pathogenic functions associated with human gut and decomposition function including aromatic compounds and plastics degradation were significantly higher in tiankeng. Further, the patterns of microbial function assemblage were different between tiankeng and karst land. Results suggested that compared with karst land, rich soil nutrients drive higher microbial diversity and function in mature tiankengs. Since the soil microbial hotspot was defined as soil with small volume and high process rates, we suggest that the theory of soil microbial hotspot may be applied on landscape scales, i.e. mature tiankeng is the soil microbial hotspot in karst region, because their small volume proportion but high soil nutrients, microbial diversity and functions in karst region. We further suggest identifying the soil microbial hotspots in other landscapes and this may be key to biological conservation and ecosystem stability with global changes.