Soil depth governs microbial community assembly and enzymatic activity in extreme environments

Aim A fundamental challenge in soil macroecology is to understand how microbial community structure shapes ecosystem functions along environmental gradients of land surface (i.e., horizontal dimension). However, little is known of microbial community structure-function relationships along environmental gradients of soil depth (i.e., vertical dimension) in extreme environments. A full understanding of the consequences of environmental change for microbial communities structure and subsequent changes in microbial functions could enable more accurate predictions of extreme environmental change effects. Here, we leveraged a 200-km desert soil salinity gradient that is created by a 12-year saline-water irrigation to evaluate how soil microbial community structure-function relationships change with soil salinity in the horizontal and vertical dimensions. Location The Tarim basin of Taklamakan desert. Taxa Soil bacteria and fungi. Methods We assessed the prime ecological processes controlling the assembly of microbial communities and the activity of enzymes relevant to carbon, nitrogen, and phosphorus cycling along soil salinity gradients across study sites (horizontal dimension) and soil depths (vertical dimension) by using the general linear model, hierarchical variance partitioning, and path model. Results Differences in soil depth (on the scale of meters) was as important as geographic distance (on the scale of kilometers) in shaping the structure of bacterial and fungal communities, while both the vertical and horizontal variability in enzymatic activity were largely attributed to the increase in the heterogeneity of soil properties, such as soil texture, water content, and pH. Main conclusions Our results suggest that dispersal limitation and environmental heterogeneity, not soil salinization, along soil depth governs microbial community assembly and enzymatic activity, respectively. This work highlights that conservation efforts of soil macroecology should consider soil depth as a key attribute in the face of ongoing salinization in arid ecosystems. ### Competing Interest Statement The authors have declared no competing interest.